[request] Add opaque option in overrides

[Luwx]

Sometimes a opaque title bar might fit better a specific window, so having a option similar to BreezeEnhanced's would be great

[alex47]

Thanks for the feedback.
I personally only add features to this theme that I would actually use.
That being said, if someone made a pull request that adds this, I would happily merge it.

[Luwx]

Alright
My developing skills are not there yet but I could submit a patch in the future :)