This was after the classic green. Steam has basically gone through 3 major redesigns. They usually were part of some bigger product launch.

• Classic green
• Dark gray (coincided with Orange box and launch on Mac OS X)
• “Big Picture” blue (coincided with the launch of SteamOS, Steam Link, Steam Controller, and the failed Steam Machines)
• Basically KDE plasma (coincided with the launch of the Steam Deck)

This is from memory, so I could be off on some of this.