People use outer shadow everywhere.
Aurora, QulightUI, oUF_NIN, oUF_Freeb...
I think one-pixel and outer shadow are the two kinds of border found widest used.
The third goes to tuk-style.(Elvui ShestakUI AllezUI...)
The forth goes to !Beautycase-style(oUF_Phanx LerbUI Caith UI)
just personal opinion..