DigiSpark AT Tiny85 compile problem?
The problem is the following:

The digispark core already implements the TIMER1_OVF_vect ISR (Timer1 Overflow Interrupt). Since you have a 1 for the #define TIMER_TO_USE_FOR_MILLIS, Timer1 will be used for counting the millis() value in the background. For this the Timer 1 Overflow interrupt is needed.

The code, that you copied from that site, does not anticipate, that you might use a core, that already implements the interrupts. It is written for a generic ATtiny85, not specifically for the Digispark version.

What you can do to solve the problem:


You can use Timer0 for your code, keeping the Digispark core untouched. You can change all registers from Timer1 to Timer0 (by replacing the 1 with a 0). Additionally you need to replace TCCR1 with TCCR0B, because for Timer0 that's the register, that set's the prescaler (and turns the timer off if zero is written to it). Also you need to consider, that Timer0 has other prescaler settings. So the line TCCR1 = B00001011; in zeroCrossingInterrupt()  must be changed to TCCR0B = B00000101;. That will set the prescaler to 1024 as mentioned in the comment. The resulting code compiles fine for me with the digispark core (though I have not tested it's functionality).
You can set the Digispark core to use Timer0 for millis(), by changing the definition of TIMER_TO_USE_FOR_MILLIS in the Attiny85 section of the file arduino_folder/packages/digistump/hardware/avr/1.6.7/cores/tiny/core_build_options.h to the value 0 instead of 1. If you do this, the original sketch compiles fine. Though the comment before that build options states, that Timer1 is better for millis() "for various reasons", which is not very specific. You can try, if it works good for you.
You can use another Attiny core. Though you might have to change it too, to accomodate for the unusual clock frequency fo 16.5MHz (most cores I have seen have 16, 8 and 1MHz). That might be as easy as changing the value of the defined clock speed in the cores files, but I'm not sure about that.