So, I have an Arch Linux install with plasma installed, and I installed Arc Dark and it replaced all panels blurry-transparent (and everything that's supposed to be white/black with Breeze).
I tried everything: from reinstalling plasma and deleting the theme, to deleting files in /usr, ~/.local, ~/.config, and more.
Now it not only still has the blurry-transparent effect, but it also doesnt have background.

If you can help me I would really appreciate it. Thanks.

Well, I think I'm just going to reinstall Arch and reinstall KDE Plasma....

Execpt you can help me... Please ;-;

makmm wrote:I tried everything: from reinstalling plasma and deleting the theme, to deleting files in /usr, ~/.local, ~/.config, and more.

Well, deleting random files (especially system files in /usr/) is unlikely to fix things and actually rather a good way to break your system even more...
And reinstalling won't change any user settings.

Have you tried to just switch back the Plasma/Desktop Theme to "Breeze"? (Systemsettings->Workspace Theme->Desktop Theme)
Or try to change the "Look and Feel" (in Systemsettings->Workspace Theme->Look And Feel), that should set all theme related settings at once.

If that doesn't help, try to delete the theme cache, i.e. the files ~/.cache/plasma* and logout/login.

Regarding the background: right-click on the desktop or left-click on the toolbox icon in the upper-left corner, and choose "Configure Desktop", you should be able to set any wallpaper you like.