It's honestly not quite as simple as just 'swapping' something in or out. An MC skin is literally a house of cards. Change one thing always affects something somewhere else.
Marko is downplaying the effort involved - creating a skin is a monumental task. Granted, having all the existing image elements completed where one "only" needs to modify colors does lessen the work involved, but it is still a huge time sink.
In the interest of saving my friend Marko's time, have you tried going to the menu bar
View/Skin/Skin Effects… yet? From there, you can adjust the brightness/contrast levels of any skin. It can take a bit of tweaking/experimenting with those controls to get the skin to both look good and still be readable, but if your patient, you can generally come up with a combination that you
may like better than the original skin.
In that same dialog box, there is also an
Effects drop-down menu. Try selecting the
Invert option, which makes dark skins light, and light skins dark. Granted, there are very few skins that look good when you invert the colors - in fact most look god-awful. But there are a handful that actually look really good inverted. Unfortunately, I think
Black and Blue is one that does not look so great, which has nothing to do with the skin designer. One does not design a skin with a plan to make sure it also looks good with the colors inverted. It is only by happenstance if that turns out to be the case.
Obviously it is Marko's call if he is willing to put in the time, but as he indicated, it may be quite a while before he could complete such a task. The options I list above may get you what you need in the meantime.