Thank you, I already have that plugin LittleStain . Although I don't really see what the big difference is between using that one or just importing the file from the projects tab....
Biggest difference in my opinion is that you don't have to do anything..
Just adding it to my project made sure the css I needed to make my openstreetmap work worked..
So all you'd have to do is add an ID and reference it in your css file and it should work..
No further actions..
Adding an ID wasn't even nescessary..