Fine that you use Code Tags 
Can you add some comments to the timer initialization at the right place?
There remain undefined bits in the control registers which you never set nor clear.
Setting OCR1A (TOP) is not required in the ISR, may cause strange effects.
OCR1B increments beyond 2500 so that no more match is detected until it wraps around to 0.
This should toggle PB2 once with each change of the duty cycle, but it may do nothing if the waveform generator is activated for PWM. For testing purposes I'd use a different pin, not affected by the timer.