On OSX with the dark toolbar, the toolbar icon looks like this: .

I think the answer is to use Template Images. I tried just appending template to the pngs in /lib/assets/* as well as within app-tray.js. It worked but the icon is probably too large with a HiDPI monitor and the inverted play and paused were weird.

I'm getting ready to move to a different country but I can try to mess with it a bit this weekend if I get a couple of hours.