React to Dark/Light theme changes

[rubiojr]

Swamp supports dark and light themes, but it doesn't react to theme changes while it's running, meaning you need to restart the app to take advantage of it.

I need to research the standard way to do it (assuming there's one), and see if Gotk3 bindings have support for it.

Tentatively for 1.0, depending on the work required, though I suspect theme changes don't happen very often and it'd be fine to ship 1.0 without it.