The Skins are optimized so everyone can change the main colors. It's very simple.
You need a Code Editor like Notepad++ or Notepad.
Open the XML File and on top you will find the variables section
Just change Color="149,33,177" and DarkColor="100,10,100" values in every RGB color you prefer.
FlatUI Dark Skin
(http://i.imgur.com/dMnGLXN.jpg)
(http://i.imgur.com/Zgd3Khs.jpg)
(http://bit.ly/2kyMmGG)DOWNLOAD (http://getmusicbee.com/addons/skins/70/flatui-dark/)
FlatUI Black Skin
(http://i.imgur.com/7T1xmBQ.jpg)
(http://i.imgur.com/MsYWfW3.jpg)
(http://bit.ly/2juRZnF)DOWNLOAD (http://getmusicbee.com/addons/skins/83/flatui-black/)
FlatUI Light Skin
(http://i.imgur.com/tw6VVGe.jpg)
(http://i.imgur.com/tKIVR0F.jpg)
DOWNLOAD (http://getmusicbee.com/addons/skins/90/flatui-light/)