Yes I can confirm that even with Plasma 6.2.91 (Beta 2) and with kernel 6.13, plasma goes black basically, while Systemsettings remains open. Seems like it tries to read from ~/.config/xsettingsd/xsettingsd.conf and plasmashell (/usr/bin/plasmashell) hangs consuming 99% CPU, that’s why your fans go wrooom.
/edit: There is a thread about it already: Black Screen after applying a theme and the bug report is 498175 – Desktop goes black and panels disappear when changing the global theme with "Desktop and window layout selected", until plasmashell is restarted
So please do a better search next time so that we don’t waste time trying to reproduce and investigate something that has been done already. That is even more expected from Arch users.