feat: New "Generic" Theme

[mautamu]

Describe the feature

While we already have some basic themes available, it would be nice to have a battleproof theme that will run regardless of what's installed. E.g. choose polybar/xmobar/lemonbar/eww, etc, based on what's available and use feh if it's available, and so on.

This would then allow packagers to link to this so folks are dropped into this default theme rather than a black screen.

de soluiboi on Discord in #leftwm channel.

How important is this feature to you?

It is just an idea

How important do you think is this feature for the average LeftWM user?

Would probably be nice to have