In #3736 the meaning of Window::theme() was changed to not return the system theme, ergo returning None without a theme override through Window::set_theme().
This is how currently MacOS works, but not how Web and Windows do.

I propose the following change:

• The meaning of Window::theme() should be changed to return the current window theme, if no override is in place it returns the system theme. I think a use-case for only returning an override has yet to be observed.
• MacOS should return the system theme in Window::theme() if no theme override is in place.
• A new ActiveEventLoop::system_theme() method should be introduced that returns the system theme.
• (optional) A new Window::system_theme() convenience method should be added that returns the same as ActiveEventLoop::system_theme().

Cc @madsmtm.