Support to set theme by API

[embbnux]

Currently the embeddable only support to set dark them in user's settings.
Developer would like to change it by some API to follow host website's theme.

Maybe: ?theme=dark in query params.

Current quick solution:
When the widget's theme setting is set to follow system setting, developer can change the widget's them by set CSS of the widget iframe:

iframe {
  color-scheme: dark;
}