Hello,
I already found the solution for my problem (this is an edit of my original post), but for future reference, here's the problem and the solution:
Problem:
"Basicaly CSS works, but it has a (very anoying) problem. In the menu's and during gameplay the whole game-screen is flickering with black triangles. It's very rough on the eyes to look through it, but when I do try to look through it and play a game then the gameplay and the fps seems allright! The black triangles do not seem to be attached to any geometry (so they are not polygons from the game world)."
Solution:
In KDE options I had enabled some 'Desktop Effects' they are not anything like Beryl/Compiz so I totally forgot them. I disabled them and now it's all fixed.