Define the flyout brushes in the theme dictionary

Previously in #1218, I had the problem that we couldn't figure out which theme is the inverse version of a custom theme when using `Inverse` in flyouts. ~~We simply solve this by preventing the usage `Inverse` when not using either `BaseDark` or `BaseLight`~~ 
We rely on the theme being named "Themename" + "Light" or "Dark"

This PR effectively removes the Theme class from use (we keep it for compatibility reasons) and we are more flexible when it comes to flyout theming