One of the areas of this program that I really like is the button bar at the bottom. A great idea where you can easily assign your own commands to buttons. Brilliant! However the look of it is a bit "blah". No offense. Even providing the ability to resize the buttons or assign icons would make it more attractive.
If you are given an icon library that handles all the icons within the program itself as well as any extra commands that the user designs, then choosing one of those to be the button face would be good.
I imagine however that it might be a bit more of a rewrite for most of the interface to read the icons from a user defined icon library so maybe just the buttons on the bottom.
Just now as I am writing this post I was also looking at the configuration options in MC and noticed something odd.
You have too many ways to change the same sort of things and they don't always match up and produce the desired effect.
1) Configuration -- Quick Look'n'Feel Setup -- Customize -- Colors -- (choose from the list)
Too many clicks and changes of interface even within this one setting. File menu to new dialog to another new dialog and then a dropdown list.
2) Configuration -- Explorer Panel Settings -- Display or Layout of Colors (choose Display and then...) Fonts... As well as these options there is a button on the bottom of this new panel to "Import" a new scheme which loads a new dialog with another dropdown list to select a scheme.
As with 1) too many different changes in interface to edit settings: menu options to dialogs to tabbed panels to more dropdown lists and buttons.
Selecting one of those schemes changed the font colors but nothing else which meant my previous layout of the Quick Look'n'Feel setup of alternating colors now had some text with same background color. Other elements also changed to undesirable styles and there is no option to restore the layout, color or style to "factory" settings. I now have to figure out which style element was changed by which option menu.
So if I could summarize what I feel the most important UI changes you should make are:
1) Reorganize the different ways to edit all the elements onto one place from which ALL the settings are easily identified and changed. Don't have multiple ways to do the same thing for this sort of thing, it is something the user may do once in a while and doesn't need to be in a quick one click setup but it should also be presented in a unambiguous manner so as not to confuse simple users. Like me.
If these settings could be saved into some sort of config file then you may find users could share their settings and you might be able to include them in a list of easily defined settings, in the same way most code editors have different layout styles.
2) Pretty up the button bar with the ability to change the size and maybe adding icon support.
3) If ALL the icons, even those in the main program, were loaded from an icon library (some *.dll maybe) then users may also be able to load different libraries if so desired.
This last one is a really low issue one though. 1 and 2 really need attention in my opinion.