Here is an official Microsoft skin for Windows Media Player. Its called "Brilliant Black". Actually this skin is a part of Microsoft Touch Pack for UMPC OEM (Ultra Mobile Personal Computer, Codenamed Origami).
It works in Windows XP and Vista and Windows 7 too. You can use it in WMP 9 and higher versions.
Just download the ZIP file, extract it and double click on "Brilliant Black.wmz" file. It’ll automatically install and apply the skin.