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 :)

La issue cita `cmdbar_bg`, i file di configurazione dei temi e `struct_patch`; inizia tracciando dove `cmdbar_bg` viene definito e applicato. Confronta gli approcci proposti basati su `Option` e su un’opzione booleana, quindi verifica che un tema possa disabilitare il riempimento mantenendo l’aspetto predefinito.

Scritto dal modello di indicizzazione a partire dal testo della issue.