Radioshack's theremin circuit - how does this work?
It's not a real theremin. This circuit creates a 'tune' based on the amount of light that hits R2. It's a simple design based on the well known 555 timer. Assumed in bright light the R2 value decreases to 1k ohm. THe output is between 700Hz and 5kHz. 

A theremin is a similar principle with a RF circuit instead of light.

http://www.ohmslawcalculator.com/555-astable-calculator 

with: C = 100nF, R1 = 1k Ohm, R2 = 1k ohm to 10k ohm. 

https://en.wikipedia.org/wiki/Theremin