(This is a half-rant half actual question)
I wanted a nice qt theme to use on Scribus since Arc Dark doesn’t work on there. When I tried to install it, pacman said this will install 50 packages. 300 Mb in total.
Why does a theme need packages such as Kauth and Kwallet?
Exactly. Even on Gentoo, which is parsimonious about dependencies, installing breeze pulls in a good chunk of KDE (although you can get just the icons as a separate package). I imagine it includes window decorations and such which need to be compiled against some of the KDE base code, and that’s pulling in everything else.
I’d suggest qt5ct if you’re trying to set up theming for a few QT5 programs on a non-QT5 desktop. It provides a basic GUI for changing colours/fonts/styles/icons rather than a prepackaged theme that may contain more than you want.