Say, you like DarkRED skin a lot but would rather have a little brighter color for its header bar. Now with latest v3.1 of MB, this can be done without asking the skinner for help even with bitmap skins (the filename of which end with xmlc instead of xml).
Just create an xml text file with a text editor (like notepad), add some code, and put it in the same folder as its original skin. It's that simple.
With the example above, the code would look like this:
<?xml version="1.0" encoding="utf-8"?> <root dependsOn="DarkRED.xmlc"> <element id="HeaderBar" bg="40,40,40" fg="200,200,200" bdr="40,40,40" /> </root>
Most of the time you will only need to modify the third line (and the name of skin file in the second line). And then name the xml file like "My Skin.xml" and load that skin with MB instead of DarkRED skin.
You can do the same for xml skins because skinners frequently update their skins so it would be more convenient for you to have a dependent skin rather than to keep editing your own code every time when the skin gets updated.
One thing to keep in mind is many colors of player bar in bitmap skins cannot be overridden if it's using its own images for those elements. But with xml skins, you can control everything.
For the skinning code, consult the "Skinning Elements" page (http://musicbee.wikia.com/wiki/Skinning_Elements) on the wiki and skin developers forum (https://getmusicbee.com/forum/index.php?board=21.0). If you still can't find an element, post a question on either Questions forum or skin developers forum. Enjoy!