Add dark theme for the manual lyrics search popup

[Velocidensity]

Lyric editor and other popups already use dark theme, if enabled (on foobar2000 v2.0), but the lyrics search window does not.

[jacquesh]

The same is true for bulk search. This is because at the time that I added dark mode support, fb2k's dark mode theming didn't properly support lists that had items added after dialog creation. I could try this again some time to see if that's changed.