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` 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 :)

The issue names `cmdbar_bg`, theme config files, and `struct_patch`; start by tracing where `cmdbar_bg` is defined and applied. Compare the proposed `Option` and boolean-option approaches, then verify that a theme can disable the fill while preserving the default appearance.

Written by the indexing model from the issue text.