Add a global `bg_color` config option

[dastarruer]

Checklist

• I've looked through the documentation and existing open issues for similar feature requests.

Describe the feature request

While trying to set a background color for all widgets, I found that the text = {bg_color...} option does not add a background color for the memory, cpu, and network widgets. Instead, the background color is based on the terminal's background color.

Specifically, this is needed for nix-community/stylix#2181, as the testbed for light mode fails if a background color is not set (see this comment).

[ClementTsang]

Instead, the background color is based on the terminal's background color.

This is intentional.

That said, not sure if the effect you're getting right now is intentional lol.