[plt-scheme] possibly evil drscheme ui thought

i think it would be neat (this will presumably incur the wrath of
folks who already can sight-read parens and who will tell me that
everybody should learn that and not lean on ui crutches :-) if
matching parens were subtly background hilighted at all times, not
just when there is a cursor there. i'm not 100% sure how it would
work, but basically i think if it was black-on-white text then the
background hilight would be very light grays. and each new inner level
would be a slightly darker gray. or maybe even different pastel hues
could be utilized.