Fade a LED using fast PWM and interrupts on Arduino
If you know how to generate an interrupt on timer0, I assume you already finish the fast PWM registers setup parts. Based on mentioned documentation by @Chupo_cro you can change the brightness by change the OCR0A or OCR0B value. i.e. your setup was using timer0 - A, and it was on non inverted mode.

OCR0A =   0; //  0% duty cycle
OCR0A = 128; //50% duty cycle
OCR0A = 255; //100% duty cycle (since timer 0 is 8 bit)