How to set color of background layer?

I select a layer, then right click choose"Background from Layer", it filled by dark purple.
So, how can I set custom colors of background layer?

The current background color gets filled on the layer. (The bottom of the two color selections in the bottom left)
But you can just use the bucket tool to fill it to anything you want. There’s no real reason to use a background layer tho and you can just fill a layer with a color and it works the same, except that background layers currently have a bug causing exported animated gifs to not work properly.

In Indexed mode, background layers are the only way to get your palette colour 0 to show up without being forced to be transparent, which can be important if you need to export an image with all the colours visible but without messing up the palette order by adding a dummy colour.

…in other words, the only reason to use this buggy feature is to get around an unimplemented feature :'D