I had a custom dark theme installed, I went in and made a change to the style of the windows. when I clicked apply, system setting crashed and automatically closed. now I cant open it to make any changes al I get is the " system settings closed unexpectedly " notification.

also the software update will not open up same error message.

any help? I would really not like to reinstall the OS.

I am on Debian 11 with Plasma 5.20.5

Please file a bug report. If this is about a third-party theme you need to report it to the theme author.