Not sure if anyone else has noticed this, but it gets around one of the
more annoying limitations of using themed controls. Normally when
using themed controls a triple state check box looks the same in the
unchecked and null states - very annoying and not all that useful.
Today I found by accident that if the border style of the checkbox is
set to anything other than "solid", the check box takes on the
appearance of a non-themed control and behaves as a non-themed control
showing three different states as it should.