Toggle between Light & Dark mode.

Toggle macOS Theme.py

#!/usr/bin/python
#
# This toggles the macOS appearance from Light mode to Dark mode and vice
# versa.
#
# Big thanks to Mike Lynn for his help. Esp. this gist:
#   https://gist.github.com/pudquick/ba235b7e90aafb9986158697a457a0d0
#
# Still looking for the function to perfectly simulate changing themes via the
# System Prefs UI. iStat menus seems to not be getting the hint.

import platform
from Foundation import NSBundle

vers, _, _ = platform.mac_ver()
if float(vers.split('.')[1]) != 14:
    exit
else:
    appearanceBundle = NSBundle.bundleWithPath_("/System/Library/PreferencePanes/Appearance.prefPane")
    appearanceShared = appearanceBundle.classNamed_('AppearanceShared')
    app = appearanceShared.sharedAppearanceAccess()
    if app.theme() == 1:
        app.setTheme_(0)
    else:
        app.setTheme_(1)

I don't know of a way to enforce themes, yet. Hence, digging under to hood to get some additional access to them.

Using these methods and some kind of Restrictions profile payload to lock down System Prefs may be the best?

Doesn't notify apps when run from a different user context. Even sudo -u "$username from the root user doesn't send the notification to apps.
Yeah. This is a decent start, but needs some fluff for EUX.