Game windows
Character, spellbook, quest and other supported windows share dark backgrounds, gold titles, thin borders and matching close buttons.
The close button and the tabs take the same flat style. The selected tab is lit; the others stay dim until you hover them. Each window's content sits in a darker inset.
The contents and controls of each window keep their usual purpose. See Window interiors for the different panels and Auction house for the auction layout.
Restore the original frames
Disable Panels under Settings → System → Modules and reload. Features with their own module can be turned off separately, and Settings → Interface → Windows turns the styling off for one family of windows at a time.