Transparent background can enable system-like looking UI (see screenshot in #647).

In this issue we also should come to consensus if transparent should be a default or it should be enabled via config.
IMO it should be default and white background should only be shown if user has set the body background color to white.

This is a good first issue for anyone who wants to participate. Let's come to the conclusion and then I or anyone else from the team can describe what changes are needed to make this happen