This is a custom team selection screen that works on all aspect ratios (16:9, 16:10, 4:3) in StarCraft 2. It is free to download and modify in any way. This doesn't include the background image as it was created specifically for my custom map. If you are interested you may request one for your map and I will create it if you include my name in the credits.
It can be used for team selection at the start of a game or even to create new teams midway through the game.
Here is a video showing it's functionality:
and if you wish to understand it, 2 videos showing how it works: