Update theming docs for new theme + color scheme preferences (JabRef/jabref#15798)

[koppor]

🤖 Generated with Claude Code

JabRef/jabref#15798 reworks theming: the preferences now offer a Theme (JabRef, Primer) plus a Color scheme (Follow system / Light / Dark), and a custom CSS file is applied on top of the selected theme instead of replacing it.

Affected pages include Custom themes and any screenshots of Preferences → General → Appearance. Docs should be updated once that PR is merged.

[Maran23]

Fixed by #662 ?