theme_importer: Fall back to inactive tab background if no active tab background set (#3598)

This PR makes it so the `theme_importer` falls back to the inactive tab
background color if the theme doesn't not an active tab background
color.

This fixes the active tab color in the Synthwave 84 theme.

Release Notes:

- N/A
3120a9cbeaMarshall Bowers committed on 12/11/2023, 10:39:46 PM· committed by GitHubparent050ff6b
2 files changedLine totals unavailable