My custom XFWM theme has square corners on windows without focus and large-radius rounded corners on the one window with focus.

The square corners are part of a 2 pixel wide border (one black, one white) because who needs to waste space on handling things we aren't manipulating? But the title bar is high-contrast, because you'll go looking for it when you want to switch windows.

The round corners go with a fairly thick border in a customizable color, usually something very bright in the yellow, orange or cyan ranges. When you sit down, you should immediately know what is active.