Any way of customising the skin without directly editing the CSS would
be extremely limited. If you make a list of things you would like to
be easily customisable, it shouldn't be difficult to knock up a
special page to do it. However, you would probably have to choose
between customising via the special page *or* by the CSS - I can't
think of a practical way to merge customisations from both (except for
using the special page as a starting point and then editing the CSS,
but you would lose any edits to the CSS if you changed anything on the
special page later on).