[thx4nothing]

Is it possible to force an aspect ratio for a skin?
I have an ultra wide monitor and every artwork or menu is stretched from 16:9 to 21:9. (Playback is fine)
It would be nice to have black bars on the side to make everything look normal.

[Klojum]

I have an ultra wide monitor and every artwork or menu is stretched from 16:9 to 21:9. (Playback is fine)

Kodi's default skin is 21:9 aware, so the GUI layout will be adapted to the different aspect ratio.
Artwork is not stretched AFAICR, it is merely zoomed, so background posters will have a bit of top & bottom cut off.
If all artwork is stretched, perhaps you are currently using a different, 21:9-incompatible skin?

I suppose a skilled skinner would be capable of displaying Kodi's GUI in a 16:9 box. With black bars...
I'm not sure if you can persuade background images to move or bend just as easily.

[thx4nothing]

Ah yes, I should have mentioned that I'm using a skin (Aeon Nox Silvo).

I thought it might be possible to edit the files of the skin a little to accomplish my goal. I read somewhere about "ratio=keep", but maybe it doesn't help.