1. Click the RoRetro button.
2. Find Theme and pick a classic era from the list. Off means today's normal Roblox.
3. Go to your Roblox tab and press F5 to refresh it. The whole site now has the classic look.

Something looks half-changed? Refresh the page (F5). That fixes it almost every time.

More eras are being built. You'll see them in the list as soon as they're ready.