There's no difference between using and .SRT and .Advanced Substation Alpha (A.S.S) files on Kdenlive editor.
When I import a .Advanced Substation Alpha (A.S.S) subtitles with a background, the editor doesn't import the background with it.
Also there's no way to change subtitles' fonts

Kdenlive version: 21.12.3

OS: Windows 10

Usually such imports can be edited. (Not able to double check now)
In the usual text editor one can set these kind of wishes. In the usual text editor there's usually a select box ☑ to shut down or activate such background color.
So i would start with double clicking this subtitling file in the Project files top left. I expect this text editor will open then.

I'm disappointed the newer version of Kdenlive didn't include a subtitle editor.

Nothing changed

https://www.youtube.com/results?search_ ... +subtitles