I have been suspecting that the GeneratePress puts the style code in some sort of general WordPress styling file since it has cross-generated between themes.
Definitely not – GeneratePress adds styling inside its own stylesheet, and into the wp_head section. No core WordPress files are changed (not even sure this is possible).
The add-ons should be deactivated if GeneratePress isn’t activated – they’re not meant to be used with any other themes. They’ll still show up if GP isn’t activated, as they will run their code regardless since they’re plugins. This may be the cause of some of your confusion.
So, try deactivating the add-ons and switching themes. This will make it impossible (literally, as none of the code can be or is read by WordPress if the plugins/theme is deactivated) for anything GP related to carry on to other themes.
Hope this helps 🙂