well, thanks for the answer. But I've tried that already :)

The file is the templates/popup_trans.html. But the same problem: it's the same file for the popup menu as the popup to upload media. So if I change one background color it also changes the other. As one has light color for font and the other dark one of them is always unreadable. Same is with the buttons, it's class bx-btn so it changes all colors everywhere. 

I need one class just for the popup menu best.