Step 1
Create a simple 400px by 400 px square, with 15 px corner radius.
Step 2
Create a radial gradient on the square (with handles set as shown below), where both color stops are 95% opaque.
Step 3
Choose 2 shades of color to use. Basic color theory states that lighter shades of color are “warmer”, and darker shades are “cooler”. In this case, green is used, but you may use any hue. Make sure each color’s shade isn’t too far apart from each other.