Target (23/06/2025) - tied for 2nd place

Solution (135 characters):
Breakdown:
This was achieved with the html and body elements only. The green outer area is achieved with a box-shadow on the html element. The red and yellow areas were achieved with a radial gradient on both elements. A border radius is given, then some margin for positioning. Pretty straightforward.
Interestingly, out of the top 10 the only solutions had either 134 or 135 characters. All the solutions had the exact same rules, but the players with 134 chars where a bit more succinct with their selectors.
The use of box-shadow may not be so intuitive this time, but the reason it is used is because the background/ background color property was already in use, and using outline or border instead would have prevented the overlap of the body element from being visible.
Here are the styles, unminified: