EDIT: Never mind! I got it now, just didn't saw the solution you posted, because I was confused. Thank you! Just use the already existing configs folder through Samba and add the themes folder manually in emulationstation. It works, thousands times thank you.

@dudleydes I just don't know how to do this. How can I create the connection to this folder through Samba? Searching this gets me confusing, I just can't get around it. In Nautilus, I use the smb://retropie/ path to get a selection of 5 folders to access from: bios, configs, print$, roms and splashscreens. I don't know if I added one of them manually and if I did, I don't remember how. I wish there would be a sixth folder to access, the theme folder. Can I add it there?

@dudleydes

Yeah, you're right. Just simply shut down the Pi once and swap the keyboard with the WiFi dongle and vice versa is the more comfortable way to do it without the need to hurry.