From what I've seen I might need a KDE theme. I have found this, but it gives me a .qtcurve file.
I looked up how to use it and this page told me to use KDE system settings. Sounds weird, but ok, I installed the systemsettings package.
But in the KDE system settings I only had the categories "shortcuts", "network settings" and "network connectivity". So I installed kde-config-gtk-style and it did indeed add the "application style" category to the KDE settings.
There I first tried setting "BlackMATE" as the GTK2 and GTK3 theme, which did nothing. Then I tried importing the downloaded theme file, which didn't work, because it expected a .tar file.
So I packed the theme file into a .tar archive (which seems weird, why would I need to do that?) and imported that, which made the settings window become unresponsive and then close itself. I guess it's not the sort of tar file it expects?
When I click "download GTK2 themes" or "download GTK3 themes", it stays at "initialising" and does nothing.