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?
When you're not telling us which package you're trying to install in which packaging system, the only meaningful answer is: you're trying to install the wrong package.
I'd check that you're actually installing the most appropriate package. For instance on Ubuntu there's kid3 which is a MP3 tag application that will pull in the entire k desktop environment. Or you can install kid3-qt which packages its own version of those dependencies and doesn't pull an entire desktop environment in if you're using a non-kde environment.
it's one of those packages that are only put in the repo with the intent on being itself a dependency of the full kde desktop, since it's a component of the deskop and not just a random theme
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.
Breeze is artwork, styles and assets for the Plasma desktop. I have seen this happen with other distributions as well. I couldn't tell you why it needs all of those extra bits and pieces.
This is one of those areas where Flatpak shines - install the Flatpak versions of Scribus and Breeze, and keep your system free from unwanted package bloat.
If you use Flatpak from the start, the storage thing becomes less of an issue.
Flatpak only takes considerably more space because people use Flatpak as a last resort or too late into the life of the current installation, as flatpak will have too many requirements for too little apps.
flatpak has gtk breeze no the theme for qt apps and in general I don't see how this would help non flatpak applications which aren't going to be looking in some flatpak dir for themes.