A script to toggle darkmode in RoboFont, for app UI and Font View. Also includes a dark theme for the RoboFont Theme Editor.

00-toggle-darkmode.robofont.py

"""
    Toggle a dark mode in RoboFont.
    
    For a dark mode in the Glyph Edit view, install Theme Editor: 
    https://github.com/connordavenport/Theme-Manager
    ...and import the "Dark Connor" theme from this Gist 
    (https://gist.github.com/arrowtype/ce0cee426f47c2fe7e82aa07d1e0a450).
    
    Also offers to edit the markColor of glyphs with a high opacity in their markColor, 
    which will disrupt the visual effect of a dark mode on the Font View. However, 
    this only works for currently-open UFOs, and will edit your glyf data, so you may wish 
    to answer "No" to this option (or remove it from your copy of the script).
"""


# ------------------------------------------------------------------------------
# Toggle app UI dark mode. Thanks to @gferreira at:
# https://forum.robofont.com/topic/565/activating-dark-mode-in-macos-10-14-rf-3-2?_=1588526372627

import AppKit

aqua = AppKit.NSAppearance.appearanceNamed_(AppKit.NSAppearanceNameAqua)
dark = AppKit.NSAppearance.appearanceNamed_(AppKit.NSAppearanceNameDarkAqua)

if AppKit.NSApp().appearance() == aqua:
   appearance = dark
elif AppKit.NSApp().appearance() == dark:
   appearance = aqua
else:
   # custom appearance
   appearance = aqua

if appearance:
   AppKit.NSApp().setAppearance_(appearance)


# ------------------------------------------------------------------------------
# toggle fontview dark mode
   
from mojo.UI import getDefault, setDefault, preferencesChanged

defaultFontView = {
    # Header line separator color
   "glypHCellHeaderLineColor": [0, 0, 0, 0.2],
   # Alternative text color
   "glyphCellAlternateTextColor": [1, 1, 1, 1],
   # Background color
   "glyphCellBackgroundColor": [1, 1, 1, 1],
   # Glyph changed color
   "glyphCellChangedColor": [0.3, 0.3, 0.3, 0.8],
   # Glyph changed highlight color
   "glyphCellChangedHighlightColor": [0.4, 0.4, 0.4, 0.8],
   # Glyph color
   "glyphCellGlyphColor": [0, 0, 0, 1],
   # Default glyph layer color
   "glyphCellGlyphLayerDefaultColor": [0.5, 0.5, 0.5, 0.7],
   # Header color
   "glyphCellHeaderColor": [0.6, 0.6, 0.6, 0.4],
   # Header highlight color
   "glyphCellHeaderHighlightColor": [0.7, 0.7, 0.7, 0.4],
   # Layer indicator color
   "glyphCellLayerIndicatorColor": [0.5, 0, 0.5, 0.7],
   # Metrics color
   "glyphCellMetricsColor": [0, 0, 0, 0.08],
   # Metrics line color
   "glyphCellMetricsLineColor": [0, 0, 0, 0.08],
   # Note indicator color
   "glyphCellNoteIndicatorColor": [0.5, 0.5, 0.5, 1],
   # Skip glyph export indicator color
   "glyphCellSkipExportIndicatorColor": [0.9, 0.17, 0.3, 1],
   # Template glyph cell background color
   "glyphCellTemplateGlyphBackgroundColor": [0.9, 0.9, 0.9, 1],
   # Template text color
   "glyphCellTemplateGlyphTextColor": [1, 1, 1, 1],
   # Template text shadow color
   "glyphCellTemplateGlyphTextShadowColor": [0.6, 0.6, 0.6, 1],
   # Text color
   "glyphCellTextColor": [0.22, 0.22, 0.27, 1],
   # Unicode replace character for template glyphs
   "glyphCellViewUnicodeReplacement": "✍",
   # Collection background color
   "glyphCollectionBackgroundColor": [0.6, 0.6, 0.6, 1],
   # Collection grid color
   "glyphCollectionGridColor": [0.6, 0.6, 0.6, 1]
}

darkFontView = {
	# Header line separator color
	"glypHCellHeaderLineColor": [0, 0, 0, 0.5],
	# Alternative text color
	"glyphCellAlternateTextColor": [0.15, 0.15, 0.15, 1],
	# Background color
	#"glyphCellBackgroundColor": [0.07, 0.07, 0.07, 1],
	"glyphCellBackgroundColor": [0.07, 0.07, 0.07, 1],
	# Glyph changed color
	"glyphCellChangedColor": [0.83, 0.58, 0, 1],
	# Glyph changed highlight color
	"glyphCellChangedHighlightColor": [1, 0.69, 0, 1],
	# Glyph color
	"glyphCellGlyphColor": [1, 1, 1, 1],
	# Default glyph layer color
	"glyphCellGlyphLayerDefaultColor": [0.7, 0.7, 0.7, 0.7],
	# Header color
	"glyphCellHeaderColor": [0.15, 0.15, 0.15, 1],
	# Header highlight color
	"glyphCellHeaderHighlightColor": [0.18, 0.18, 0.18, 1],
	# Layer indicator color
	"glyphCellLayerIndicatorColor": [0, 0.36, 1, 0.7],
	# Metrics color
	"glyphCellMetricsColor": [0, 0, 0, 0.08],
	# Metrics line color
	"glyphCellMetricsLineColor": [0, 0, 0, 0.08],
	# Note indicator color
	"glyphCellNoteIndicatorColor": [0.5, 0.5, 0.5, 1],
	# Skip glyph export indicator color
	"glyphCellSkipExportIndicatorColor": [0.9, 0.17, 0.3, 1],
	# Template glyph cell background color
	"glyphCellTemplateGlyphBackgroundColor": [0, 0, 0, 1],
	# Template text color
	"glyphCellTemplateGlyphTextColor": [0.28, 0.28, 0.28, 1],
	# Template text shadow color
	"glyphCellTemplateGlyphTextShadowColor": [0, 0, 0, 1],
	# Text color
	"glyphCellTextColor": [1, 1, 1, 1],
	# Unicode replace character for template glyphs
	"glyphCellViewUnicodeReplacement": "✐",
	# Collection background color
	"glyphCollectionBackgroundColor": [0.05, 0.05, 0.05, 1],
	# Collection grid color
	"glyphCollectionGridColor": [0, 0, 0, 1],
	# Collection grid color
   "glyphCollectionGridColor": [0, 0, 0, 1]
}
        
currentDefault = list(getDefault(list(darkFontView.keys())[0]))
darkSetting = [float(num) for num in list(darkFontView.values())[0]]

if set(currentDefault) == set(darkSetting):
    for pref in defaultFontView:
        setDefault(pref, defaultFontView[pref])
else:
    for pref in darkFontView:
        setDefault(pref, darkFontView[pref])
    
preferencesChanged()  


# ------------------------------------------------------------------------------
# toggle faded markColors

from mojo.UI import AskYesNoCancel

highOpacityMarks = False

for f in AllFonts():
    for g in f:
        if g.markColor and [c for c in g.markColor][3] >= 0.6:
            highOpacityMarks = True

if highOpacityMarks:       
    changeMarks = AskYesNoCancel("\
\    Do you wish to change high-opacity mark colors to to let dark theme work ideally? \
\    Note: this edits the .glyf files in the current UFO, but only needs to be done once.")

try:
    if changeMarks == True:
        markOpacity = 0.5
        
        for f in AllFonts():
            for g in f:
                if g.markColor:
                    if [c for c in g.markColor][3] >= 0.6:
                        newColor = tuple([c[1] for c in enumerate(g.markColor) if (c[0] < 3)] + [markOpacity])
                        g.markColor = newColor
except NameError:
    pass
   

Dark Connor.roboFontTheme

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>glyphViewAlternateFillColor</key>
	<array>
		<real>0.0</real>
		<real>0.0</real>
		<real>0.0</real>
		<real>0.6027</real>
	</array>
	<key>glyphViewAnchorColor</key>
	<array>
		<real>1.0</real>
		<real>0.0</real>
		<real>0.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewAnchorTextColor</key>
	<array>
		<real>1.0</real>
		<real>0.0</real>
		<real>0.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewBackgroundColor</key>
	<array>
		<real>0.2298</real>
		<real>0.2408</real>
		<real>0.2707</real>
		<real>1.0</real>
	</array>
	<key>glyphViewBitmapColor</key>
	<array>
		<real>0.3903</real>
		<real>0.3903</real>
		<real>0.3903</real>
		<real>1.0</real>
	</array>
	<key>glyphViewBluesColor</key>
	<array>
		<real>0.1254</real>
		<real>0.9809</real>
		<real>0.9693</real>
		<real>0.1147</real>
	</array>
	<key>glyphViewComponentFillColor</key>
	<array>
		<real>0.1484</real>
		<real>0.9538</real>
		<real>0.6962</real>
		<real>0.313</real>
	</array>
	<key>glyphViewComponentInfoColor</key>
	<array>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewComponentStrokeColor</key>
	<array>
		<real>0.0</real>
		<real>0.0281126968503937</real>
		<real>1.0</real>
		<real>0.6860619288511749</real>
	</array>
	<key>glyphViewContourIndexColor</key>
	<array>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewCornerPointsFill</key>
	<array>
		<real>0.9900003671646118</real>
		<real>0.47194594144821167</real>
		<real>0.032503124326467514</real>
		<real>1.0</real>
	</array>
	<key>glyphViewCornerPointsStroke</key>
	<array>
		<real>0.9868893623352051</real>
		<real>0.1778077334165573</real>
		<real>0.02847931534051895</real>
		<real>1.0</real>
	</array>
	<key>glyphViewCurvePointsFill</key>
	<array>
		<real>0.0</real>
		<real>0.7527</real>
		<real>0.8563</real>
		<real>1.0</real>
	</array>
	<key>glyphViewCurvePointsStroke</key>
	<array>
		<real>0.4973</real>
		<real>1.0</real>
		<real>0.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewEchoStrokeColor</key>
	<array>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
		<real>0.6328</real>
	</array>
	<key>glyphViewFamilyBluesColor</key>
	<array>
		<real>0.1331</real>
		<real>0.9816</real>
		<real>0.9394</real>
		<real>1.0</real>
	</array>
	<key>glyphViewFillColor</key>
	<array>
		<real>0.0</real>
		<real>0.0</real>
		<real>0.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewGlobalGuidesColor</key>
	<array>
		<real>0.0</real>
		<real>0.0</real>
		<real>0.998193621635437</real>
		<real>0.4</real>
	</array>
	<key>glyphViewGridColor</key>
	<array>
		<real>0.7187</real>
		<real>0.7187</real>
		<real>0.7187</real>
		<real>1.0</real>
	</array>
	<key>glyphViewHandlesQuadStrokeColor</key>
	<array>
		<real>1.0</real>
		<real>0.0</real>
		<real>0.0</real>
		<real>0.8</real>
	</array>
	<key>glyphViewHandlesStrokeColor</key>
	<array>
		<real>1.0</real>
		<real>0.923</real>
		<real>0.6066</real>
		<real>0.2171</real>
	</array>
	<key>glyphViewHandlesStrokeWidth</key>
	<integer>2</integer>
	<key>glyphViewImageInfoColor</key>
	<array>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewLocalGuidesColor</key>
	<array>
		<real>1.0</real>
		<real>0.0</real>
		<real>0.0</real>
		<real>0.3986247859589041</real>
	</array>
	<key>glyphViewMarginColor</key>
	<array>
		<real>0.0215</real>
		<real>0.0021</real>
		<real>0.0025</real>
		<real>0.1083</real>
	</array>
	<key>glyphViewMeasurementsBackgroundColor</key>
	<array>
		<real>0.8245</real>
		<real>0.875</real>
		<real>1.0</real>
		<real>0.1953</real>
	</array>
	<key>glyphViewMeasurementsForgroundColor</key>
	<array>
		<real>1.0</real>
		<real>0.2</real>
		<real>1.0</real>
		<real>0.7340384301566579</real>
	</array>
	<key>glyphViewMeasurementsTextColor</key>
	<array>
		<real>0.9086</real>
		<real>0.863</real>
		<real>1.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewMetricsColor</key>
	<array>
		<real>0.8672</real>
		<real>0.8773</real>
		<real>0.8771</real>
		<real>0.5</real>
	</array>
	<key>glyphViewMetricsTitlesColor</key>
	<array>
		<real>0.7063</real>
		<real>0.7063</real>
		<real>0.7063</real>
		<real>1.0</real>
	</array>
	<key>glyphViewOffCurvePointsFill</key>
	<array>
		<real>1.0</real>
		<real>1.0</real>
		<real>0.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewOffCurvePointsSize</key>
	<real>2.0</real>
	<key>glyphViewOffCurvePointsStroke</key>
	<array>
		<real>1.0</real>
		<real>0.6668</real>
		<real>0.2573</real>
		<real>1.0</real>
	</array>
	<key>glyphViewOffCurveQuadPointsStroke</key>
	<array>
		<real>1.0</real>
		<real>0.0</real>
		<real>0.0</real>
		<real>0.8</real>
	</array>
	<key>glyphViewOncurvePointsSize</key>
	<real>3.0</real>
	<key>glyphViewOpenStartPointsArrowColor</key>
	<array>
		<real>1.0</real>
		<real>0.0</real>
		<real>0.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewOutlineErrorsColor</key>
	<array>
		<real>0.2</real>
		<real>0.8</real>
		<real>1.0</real>
		<real>0.8</real>
	</array>
	<key>glyphViewPointCoordinateBackgroundColor</key>
	<array>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewPointCoordinateColor</key>
	<array>
		<real>1.0</real>
		<real>0.9655</real>
		<real>0.6804</real>
		<real>1.0</real>
	</array>
	<key>glyphViewPointIndexColor</key>
	<array>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewPreviewBackgroundColor</key>
	<array>
		<real>0.639</real>
		<real>0.639</real>
		<real>0.639</real>
		<real>1.0</real>
	</array>
	<key>glyphViewPreviewFillColor</key>
	<array>
		<real>0.0</real>
		<real>0.0</real>
		<real>0.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewSegmentIndexColor</key>
	<array>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewSelectionColor</key>
	<array>
		<real>0.0602104589343071</real>
		<real>0.5068462491035461</real>
		<real>0.998458981513977</real>
		<real>1.0</real>
	</array>
	<key>glyphViewSelectionMarqueColor</key>
	<array>
		<real>0.0</real>
		<real>1.0</real>
		<real>0.6</real>
		<real>0.0764523498694517</real>
	</array>
	<key>glyphViewSelectionStrokeWidth</key>
	<integer>3</integer>
	<key>glyphViewSmoothPointStroke</key>
	<array>
		<real>0.0917</real>
		<real>0.0</real>
		<real>0.8563</real>
		<real>1.0</real>
	</array>
	<key>glyphViewStartPointsArrowColor</key>
	<array>
		<real>1.0</real>
		<real>0.0</real>
		<real>0.7428</real>
		<real>1.0</real>
	</array>
	<key>glyphViewStrokeColor</key>
	<array>
		<real>0.6037</real>
		<real>0.7178</real>
		<real>1.0</real>
		<real>1.0</real>
	</array>
	<key>glyphViewStrokeWidth</key>
	<integer>1</integer>
	<key>glyphViewTangentPointsFill</key>
	<array>
		<real>0.9034</real>
		<real>0.5904</real>
		<real>0.9101</real>
		<real>1.0</real>
	</array>
	<key>glyphViewTangentPointsStroke</key>
	<array>
		<real>0.9767</real>
		<real>0.0</real>
		<real>0.9491</real>
		<real>1.0</real>
	</array>
	<key>themeName</key>
	<string>Dark Connor</string>
	<key>themeType</key>
	<string>User</string>
</dict>
</plist>

Rather than hard-coding the prefs into that first script, I was having some luck with this below. This way, you can keep tweaking the prefs in the native UI, and it will keep everything (associated with the respective theme you were editing them in...).

# ------------------------------------------------------------------------------
# toggle fontview dark mode
   
from mojo.UI import getDefault, preferencesChanged, exportPreferences, importPreferences
        
light_mode_prefs_path = '/path/to/your_lightMode.roboFontSettings'
dark_mode_prefs_path = '/path/to/your_darkMode.roboFontSettings'

if getDefault("glyphViewBackgroundColor") == [1, 1, 1, 0.0721]: # use some sort of anchor value to check which mode you're in? there's prob a cleaner way to do this...
    print("changing to light mode!")
    # set to light mode
    exportPreferences(path=dark_mode_prefs_path)
    importPreferences(light_mode_prefs_path)
else:
    # set to dark mode
    exportPreferences(path=light_mode_prefs_path)
    importPreferences(dark_mode_prefs_path)
    
preferencesChanged()