Improve support for dark themes in GTK3/matplotlib

Notes

Patch to have "light" icons when using a dark theme in GTK3.
Unfortunately the foreground color is reported as full white, which gives too bright icons?

patch

diff --git i/lib/matplotlib/backends/backend_gtk3.py w/lib/matplotlib/backends/backend_gtk3.py
index 60537cf64..ed9f755a0 100644
--- i/lib/matplotlib/backends/backend_gtk3.py
+++ w/lib/matplotlib/backends/backend_gtk3.py
@@ -4,6 +4,9 @@ import os
 from pathlib import Path
 import sys
 
+import numpy as np
+import PIL
+
 import matplotlib as mpl
 from matplotlib import backend_tools, cbook
 from matplotlib._pylab_helpers import Gcf
@@ -13,6 +16,7 @@ from matplotlib.backend_bases import (
 from matplotlib.backend_managers import ToolManager
 from matplotlib.figure import Figure
 from matplotlib.widgets import SubplotTool
+from .backend_cairo import cairo
 
 try:
     import gi
@@ -489,22 +493,39 @@ class NavigationToolbar2GTK3(NavigationToolbar2, Gtk.Toolbar):
         self.ctx.set_source_rgb(0, 0, 0)
         self.ctx.stroke()
 
+    def _icon(self, name, color=None):
+        # Use PIL.Image instead of imread to get integer values.
+        img = np.array(PIL.Image.open(cbook._get_data_path("images", name)))
+        height, width, _ = img.shape
+        img[..., :3] = ((np.array([*color][:-1]) * 255).astype(int)
+                        * (img[..., 3:] / 255))
+        argb32 = img.take(
+            [2, 1, 0, 3] if sys.byteorder == "little" else [3, 0, 1, 2],
+            axis=2)
+        surface = cairo.ImageSurface.create_for_data(
+            bytearray(argb32), cairo.FORMAT_ARGB32, width, height)
+        return Gtk.Image.new_from_surface(surface)
+
     def _init_toolbar(self):
         self.set_style(Gtk.ToolbarStyle.ICONS)
 
+        st_ctx = self.get_style_context()
+        background_color = st_ctx.get_background_color(Gtk.StateFlags.NORMAL)
+        foreground_color = st_ctx.get_color(Gtk.StateFlags.NORMAL)
+        _, _, background_v = mpl.colors.rgb_to_hsv([*background_color][:-1])
+        icon_color = foreground_color if background_v < .5 else None
+
         self._gtk_ids = {}
         for text, tooltip_text, image_file, callback in self.toolitems:
             if text is None:
                 self.insert(Gtk.SeparatorToolItem(), -1)
                 continue
-            image = Gtk.Image()
-            image.set_from_file(
-                str(cbook._get_data_path('images', image_file + '.png')))
             self._gtk_ids[text] = tbutton = (
                 Gtk.ToggleToolButton() if callback in ['zoom', 'pan'] else
                 Gtk.ToolButton())
             tbutton.set_label(text)
-            tbutton.set_icon_widget(image)
+            tbutton.set_icon_widget(
+                self._icon(image_file + '.png', icon_color))
             self.insert(tbutton, -1)
             # Save the handler id, so that we can block it as needed.
             tbutton._signal_handler = tbutton.connect(