Allow adding/customizing the border of ALL windows, rather than just the focused one

[jonseppanen]

Is your feature request related to a problem? Please describe.
In other window managers, traditionally a border is drawn around all windows and the focused one is a a different color/style.
I feel like such a similar feature would be nice for paperwm.

Describe the solution you'd like
Perhaps some style tags added to every window so we can target it with user.css, or even a color/border thickness setting in the UI.

Describe alternatives you've considered
Currently I replicate this using a forked rounded window corners extension (with rounding disabled) so that I can draw a border over all my windows. This only works on xorg due to the previously documented problems of incompatibily with the extension