WS2812B with ATMega8 and light_ws2812 Libray
Okay, i figured it out!

Just changing the F_CPU Parameter won`t really change the frequency of the internal oscillator. To really change the frequency of the oscillator you have to set the according fusebits. A great tool to find the right fusebits is this Fuse Calculator.

8MHz is the highest frequency the internal oscillator can produce and a suitable value to use the light_ws2812 libray and drive the WS2812B modules.