Basically, certain UI elements (a lot) don’t use the system theme. This occurs on all gtk themes, including Pocillo ;( Light themes are usable at least, but still don’t use the system GTK widgets in those places.
This makes UB unusable. I ran into a (similar?) problem in Manjaro with gtk3, though: https://forum.manjaro.org/t/dark-gtk3-themes-broken/109968/16 PLEASE HELP
Please dont use the system theme option. It’s there only for legacy reasons when very few themes supported budgie. It is going to be removed in future.
Wait a second.
Maybe this is me being stupid, but the above pic looks like the correct theme with the wrong colors. However, on my standard user, half the themes look shockingly like Breeze.
Here’s Pocillo-dark:
Now that I think of it, this happened right after I installed some KDE apps, which seemed to install the whole KDE. This command, sudo apt purge kubuntu-desktop^, was supposed to remove the Kubuntu task which i may have installed but just freezes after “Recommended packages”
Congrats!
But that in itself points to a bug. Those themes should never interfere with any gtk themes. I would recommend filing a bug report against those packages.