From Winamp Developer Wiki
Creating a Classic Skin: Introduction → The Base Skin → Paint the Main Window → Paint the Equalizer Window → Paint the Playlist Window → Paint the Minibrowser Window → Paint the AVS Window → Paint the Winamp 2.9/5.x Windows → Create Custom Cursors → Edit the Configuration Files → Compress to .WSZ format → Submit to Winamp.com
Skinning The Minibrowser Window (Not used in Winamp 5.x)
Note: The Minibrowser is only used pre-Winamp 2.9. But if you still want to skin it, go ahead. The Minibrowser skin is similar to the Playlist skin in many aspects. Both skins are designed to be scaleable due to the fact that most web content doesn't fit into the default viewable area.
The file you will be working with is called mb.bmp. This file contains all the graphics you need to complete in order to skin the Minibrowser. The top two rows of graphics within mb.bmp allow you to create the titlebar of the Minibrowser. The first graphic on the page is the top left corner of the Minibrowser window. This is then followed by the graphic that generates the title of the window. NOTE: this graphic is centered within the window. We then come to the little graphic that is tiled across for every time the window is scaled horizontally. Finally we come to the graphic that completes the titlebar with the top right corner of the window with he close button in it's default state. The next row is all the same graphics, however, all the images are a little dimmed to demonstrate to the end user that the window is now inactive or the window that is not in focus.