Feature Request: add config option for theme

[dandalpiaz]

By default, the widget will use the light or dark theme based on the user's system preference setting, but it's nice to be able to explicitly set the theme as well. I think this could work similarly to what is done in flask-recaptcha where are configuration option is available (e.g. TURNSTILE_THEME) and is rendered in a data-theme attribute in the code, https://github.com/mardix/flask-recaptcha/blob/master/flask_recaptcha.py#L74 .