Use custom as dark mode

[isaksamsten]

Hi!

Yet another PR from me :-).

Again, this can be stacked with #1021 and #1022.

The change introduces a configuration option which I've called use_custom_as_dark which treats toggle_dark_mode as toggle_custom_color. It plays really nicely with #1022 to allow the operating system to toggle light/dark mode but the user can configure the colors of the dark mode.

[ahrm]

Thanks, however, I think modifying the behavior of dark mode is a little confusing, so instead I added a use_custom_color_as_dark_system_theme option in https://github.com/ahrm/sioyek/commit/42c8ac9c01447e4df1dbca6be00c39b9f9e8b07c.

[isaksamsten]

That's a good solution 👍