Theremin forum is a ghost town, but you can practise The Swan with Rockmore today on Google.
That Theremin Google Doodle is too easy to play, with chromatic scale preset notes. A real Theremin is incredibly much harder to play. Unless you have a version with “Assistive pitch quantization”, which Clara Rockmore certainly didn’t have in the 1930’s. She didn’t have “Pitch Preview” either to help her along. She truly was a fantastic Theremin player, and it’s nice that Google highlights her birthday.
If you can get through the lessons, there’s a free playing instrument where you can disable “easy mode” 
. You can also change scales to major, minor, pentatonic and chromatic in any key.
Thanks for the heads-up. I didn’t notice the “Settings” button on the bottom left. It’s already much harder without the “Easy Mode” on. 
But the vibrato is still automatic… 
Although Clara was incredible, I prefer Pamelia Stickney’s style of playing, because her vibrato isn’t constant but judiciously placed and controlled. 
No worries
Agreed, the best thereministe I’ve seen is Pamelia Stickney, with an avant garde band called Blueblut. Such killer synth bass! She also had a hand in the design of the Etherwave Pro, and was musing with the current engineers about a re-issue/update.
It is approaching the 100th anniversary of the Theremin. I expect Moog to do something nice.