window.systemColorTheme conflicts with window.autoDetectColorScheme

[aeschli]

Setting 'window.systemColorTheme' to anything else than default, will cause window.autoDetectColorScheme to no longer work.

Internally it sets nativeTheme.themeSource which hard codenativeTheme.shouldUseDarkColors and prefers-color-scheme and disable the update event which we use to detect the current OS scheme event.

Solutions:
a. we disable window.systemColorTheme if window.autoDetectColorScheme is enabled. Disadvantage: This will no longer allow to redefine the color of context menus / menus and frames when window.autoDetectColorScheme is on
b. we no longer use the Electron API to detect light/dark color schemes but use CSS (`(prefers-color-scheme: light)``.
The problem here is that this will cause some rewrite. We can only get the value when we have a window, while before we could access that value in main and already initialize some window colors

[aeschli]

@bpasero FYI

[aeschli]

Looks like b is not an option as setting nativeTheme.themeSourc will define what
window.matchMedia((prefers-color-scheme: light)).matches returns

[aeschli]

Ideally Electron would untangle the API that allows to detect the OS color scheme (mode) from the configuration of the Windows color mode. @deepak1556

[aeschli]

Maybe we just rename window.systemColorTheme: auto to 'match theme'

[aeschli]

The problem with window.systemColorTheme: auto is that we can configure a theme per window.

[aeschli]

I now clarified in the settings descriptions that window.autoDetectColorScheme is ignored if window.systemColorTheme hardcodes a system color.

Also I renamed window.systemColorTheme: auto to window.systemColorTheme: matchColorTheme
(PR: #210923)