add light/dark theme with theme switch injected in page (wip)

[protozoo]

A proposal for integrating light and dark default themes, including a theme-switch button.
There are a few things to consider here:

1. In Genie apps, Quasar's css (quasar.min.css) is added after stipplecore.css. This overrides some rules from stipplecore.css. So by now I made the affected properties "!important" in stipplecore.css, but that's not ideal. Would it make sense to make the necessary changes (in StippleUI?) to reverse the load order of these two css files?
2. The theme-switch button gets injected in the page dynamically with a function I added to stipplecore.js. Maybe there's a better place/way to do this?