Hi,guys. I had the same issue and now found the answer from reddit/r/ManjaroLinux/comments/hl9oox/i_broke_kde5_settings_with_a_theme/ .
just run systemsettings5 --style=Breeze in the konsole，then the system settings open, now you can change the global theme back and apply.
the “-style” can’t be found in the output of systemsettings5 --help-all, but it works.
hope it help you.