Hi!

I'm unable to change some system fonts in KDE 5.

I changed fonts everywhere, but some fonts are still unchanged especially some headers. Check screenshot
https://prnt.sc/ncr3z8

Manjaro and Appereance looks different.

And when anti-aliasing is turned off, those fonts look ugly.

Thank you!

Did you try closing the open app after applying the font settings? I have noticed at times that open apps don't automatically update and need to be reopened. Regarding anti-aliasing, I leave it on vendor default with no other settings and they look fine. If I set it to disabled the fonts look terrible like what you show in the screenshot.