Support OS dark mode

[coughingmouse]

Dark mode is now available in all three major ARM64 desktop operating systems, Windows, OS X and Ubuntu. As such, apps that do not support this will look out of place, and Thorium is one of them.

Please support OS Dark Mode on Windows, OS X, and Ubuntu, such that the "My Books" menu is dark if the OS dark mode is enabled, and in settings, you could optionally change global theme to be white as usual and dark.

Also, the theme menu within each book would also have to include a "Default" theme, where it displays as Neutral or Dark depending on the preference on the global theme (which is dependent on OS dark mode).

[danielweck]

Related PR (old and obsolete, but contains sound architectural principles, thus why it is still open)
#799

[danielweck]

Merged issue: #1532

Problem: mismatch between the CSS Media Query prefers-color-scheme: dark and the actual color theme loaded in Thorium.

To reproduce, download the EPUB from:
https://standardebooks.org/ebooks/john-muir/my-first-summer-in-the-sierra

...and load in Thorium in an operating system that has "dark mode" activated, but with Thorium using the default "light mode" (i.e. not sepia or dark themes).

@media (prefers-color-scheme: dark)
img[epub|type~="se:image.color-depth.black-on-transparent"] {
    filter: invert(100%);
}

=>

In this case we can disable the CSS filter to see the image:

[danielweck]

Fixed in Thorium3 (system dark/light modes, and publication themes beyond neutral/sepia/night), though I suspect we will get feedback about Windows high contrast, system colour palette (including MacOS which has its own highlight preference), etc.