Feature: Theme option to make `cmdbar_bg` transparent

When using a terminal with opacity/blur, it would be nice to disable the bg fill of the cmdbar items. Currently, it looks like this:

And this is what I want:

Solution alternatives

IMO the idiomatic way to do this would be to allow cmdbar_bg: None in the theme config file, but since configs are applied via struct_patch, it would require turning the field into Option<Color> and None by default (if the default is Some(Blue), patching None won't override it).

Another way would be creating a new config option like disable_cmdbar_bg: bool (default false).

I implemented the first one in a fork, but don't know if it's the preferred alternative. I'd be happy to make a PR :)