hugo: Update CSS to match new Adwaita style

This gives the blockquotes the new libadwaita "card" style. With that change, the solid borders were dropped as well.

cc: @bertob @exalm

I would be grateful if you could give a short feedback / review. Web tech / css is not my strong point, I just copied the values from GTK inspector. Is there anything else on the site that should be updated / adapted?

Before:

After: