This is currently not possible with MIDI Designer. This feature is currently under consideration, since some use-cases are interesting (e.g., import a Kontakt instrument and then put the knobs right on top of that).
The design for MIDI Designer won't be getting an overhaul nor skins for the time being (May 2013), although I'm always considering it.
Part of the reason is that I draw all that stuff programmatically, including the glows and shadows. Note how the slider track changes depending on the width, for instance. Secondly, given the roadmap of features for MIDI Designer, the current design is not going to be getting an overhaul anytime soon (bandwidth and resources issues).
Third, the slightly homely and strange look of MIDI Designer is now part of its signature. Invasive plastic surgery is a difficult thing in an existing product. New Coke was definitely better than old Coke (
according to blind taste tests), but users had loyalty stuff going on.
Fourth, and most important, is that MIDI Designer is built for performance and future functionality. For instance, although the white lines at the side of a slider do not currently reflect the "ticks' of the control, in a future they could. So any design suggestions would have to be able to provide a variable number of ticks, even though that is not currently seen in MIDI Designer.