Decided to try some new distros and this time it's Bazzite. I can't figure out any way to make any kind of custom themes work and look as expected. I tried few themes with transparent windows from "Global Themes (Plasma 6)" category but transparency doesn't seem to work in any single one of them. I've seen general KDE suggestions to try forceblur/betterblur plugins but it requires compiling it manually and since it is Bazzite, it's quite complicated and I couldn't figure it out (if I compile it in distrobox for some kind of Fedora, will it even work for my Bazzite? if I don't use distrobox, how do I even compile it? it needs tons of extra dnf dev packages). I installed Kvantum via rpm-ostree, but turns out, it doesn't style taskbar and console (also firefox, but I could live with that since I probably can find a matching firefox theme). Any suggestions?
The panel is handled by the plasma theme. I'm not sure what's the cause of your issues. Normally transparent themes work out of the box on KDE. I'd do some testing on one of my spare computers to try and figure it out, but I'm busy the next couple days.