theme: Use a non-transparent color for the fallback `title_bar.inactive_background` (#15709)

This PR changes the fallback color for the
`title_bar.inactive_background` theme property so that the title bar
doesn't just disappear when a theme doesn't have a value set.

You're welcome, @naomieow.

Release Notes:

- linux: Changed the fallback color of `title_bar.inactive_background`
to a non-transparent value.