There are two circles on the menu button at the top right.
The background color of the inner circle is black and the background color of the outer circle is yellow.
The outer circle looks actually an ellipse or oval rather than a circle.
It’s an oval because it’s taller than it is wide. The quick and dirty way would be to set the height and weight of the button to the same siize and it should give you the shape you want.
I would recommend changing border-radius to 50% instead of using a pixel value because otherwise you’re going to be tweaking that pixel value as you tweak the height/width to your liking.