What's the difference between OCRtop and OCRpwm?
There are two Output Compare registers. To do arbitrary-frequency PWM, you use one of them to set the overall PWM frequency (OCRtop) and the second to set the "on" time (OCRpwm)
They actually have to be particular registers OCRnA and OCRnB; I just haven't looked up which one has to be which, and gave them "generic" names.