Provide in preview when uploading custom background.

[somaamos]

Usability enhancement idea:
It would be nice if the Preview and/or Custom Background image on the Themes tab had visible "guide lines" showing how the site will render on both 16:9 and 9:16 (and/or other aspect ratios) so you can get a sense of whether the background image you've uploaded is going to work correctly before clicking Apply.

[JulianPrieber]

Each theme handles this differently