This is the skybox from pl_Nippon.
- Unzip the download folder into your "tf" folder (C:\Program Files\Steam\Steamapps\YOURNAME\team fortress 2\tf).
- In the Hammer Editor, open up your map properties and change your sky to sky_nippon_01.
- I recommend using Viaduct's lighting. Too pink will wash out all the players and make it hard to tell which team is which. Viaduct's lighting is ambiguous enough to work, yet it doesn't wash out players.