There are normally config files for GTK at ~/.gtkrc-2.0 and ~/.config/gtk-3.0/settings.ini.
Qt themes are configured differently. For Qt5 and Qt6, set the environment variable QT_STYLE_OVERRIDE. If you still use Qt4 for anything, config is done in ~/.config/Trolltech.conf. Refer to the ArchWiki for help.
I personally prefer GUIs, though, so I’d recommend the following: