[feature] custom theming

[bennypowers]

It would be nice to pass css custom properties to the iframe for theming:

const iframeURL = new URL('https://webcomponents.dev/edit/PmmwJU9LjBDHlkO8WiSq');
iframeURL.searchParams.set('embed', '1');
iframeURL.searchParams.set('pm', '1');
iframeURL.searchParams.set('sv', '1');
iframeURL.searchParams.set('file', 'Hello.ts');
iframeURL.searchParams.set('style', '--wcd-background:black;--wcd-color:white;--wcd-primary:#c0fefe;');

Not a deal-breaker by any means, but would add to the fun.