Nice! I'd like to get this merged with some improvements:

• Adding a listener for changes, caching the value, and read from the cache. You could monitor the registry key HKCU\Software\Microsoft\Windows\DWM\AccentColor.
• On Windows 7, the color ends up black. EnvironmentHelper should be used to check the OS, and it should probably fall back to the current Flat theme color for now since that OS doesn't have a similar concept of an accent color.

Honestly, this should probably be the default Flat theme; we could append 'Blue' or something similar to the current theme's name. We should also move much of the accent color code into ManagedShell so that other shells can benefit.

Checking for changes can be done through SystemEvents.UserPreferenceChanged.
I'll look for a way to get it to work on windows 7 in the near future, as of now I agree on the blue fallback

By using power toys and checking registry values
The colorizationColor hex value should represent the actual colorization
accentColor returns a darker value

Also another thing
Is setting the UserPreferenceChanged listener in CairoApplicationThemeService's constructor okay?
In the event by calling SetThemeFromSettings() it changes the color to the new theme's
that is surely less expensive than checking a registry value change.
As of right now I don't think caching the value should increase performance, the color is retrieved only when the MarkupExtension is loaded, and when UserPreference_changed is triggered