If I could run the Windows 2000 UI on a modern OS I would but any recent clone/theme/etc feels too uncanny valley.
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.
(Yes, please tell me about some buggy half-compatible tiling window manager for my Mac.)