Add color_scheme and/or content_color_scheme properties

[Zorziel]

Per https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme#properties, we can set a color_scheme or content_color_scheme property to force the setting of dark mode for the panels and/or content from sites that support it.

Here's the description for color_scheme:

Optional

Determines which color scheme is applied to the chrome (for example, context menus) and content (for example, built-in pages and the preferred color scheme for web pages). Options include:

"auto" – a light or dark scheme based automatically on the theme.
"light" – a light scheme.
"dark" – a dark scheme.
"system" – uses the system scheme.

If not specified, defaults to "auto".

And for content_color_scheme:

Optional

Determines which color scheme is applied to the content (for example, built-in pages and preferred color scheme for web pages). Overrides color_scheme. Options include:

"auto" – a light or dark scheme based automatically on the theme.
"light" – a light scheme.
"dark" – a dark scheme.
"system" – the system scheme.

If not specified, defaults to "auto".

The default is "auto" for both, which seems to have been based on the tab_background_text color per #1 . Since I intended for this to be a dark theme, I propose setting it to dark or at least system.