Doesn't use my theme

[ghost]

I am on arch kde and for some reason it doesn't use my theme

[Screwtapello]

That's bsnes-hd, which is a separate project, so in future you should probably report issues to https://github.com/DerKoun/bsnes-hd

However, by default bsnes uses GTK+2 for its user-interface, so it will be limited to whatever GTK+2 compatibility your KDE theme provides (possibly none). If you build bsnes from source, you can change the usual make -C bsnes command to make -C bsnes hiro=qt5 to make it use Qt5 for its user-interface (the same library KDE uses) in which case it'll probably fit better with your theme.