Support FreeDesktop dark mode preference, ship appropriate light/dark styles for text diff foreground / background colors

Originally filed as Bugzilla #788454 by @jfft :

So this was initially an issue with bug #750803, which got fixed, and then got re-fixed in bug #765727... but now the issue is that GNOME has removed the global dark theme "variant" switch from GNOME 3.26 from a users' perspective, and from what I realized by discussing with folks in #gnome-design is that we're now meant to have real separate dark themes; so in 3.26 to switch to dark mode, you have gnome-tweak-tool (or cron...) switch the GTK theme from "Adwaita" to "Adwaita-dark".

The problem is that gitg doesn't support that properly, as you can see in the attached screenshot...

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2023 update: as per the comment below, this should use the FreeDesktop dark style API, to detect light/dark dark themes switches, and use the appropriate theme and diff colors.