Add radical background color to radicals in Radical Combinations panel

[wrex]

See #119 (comment)

[wrex]

Aha.

I found out why WK had an explicit width and height on these. Apparently they sometimes use an image rather than a unicode character for the radical. Without an explicit width and height it blows up to the size of the image (guess which one of the three is an image):

(This is a review for 俵)

This needs a new rule:

#additional-content ul li span img {
  width: 2rem;
  height: 2rem;
  padding: 0;
}