This can be changed by editing the skin with Resource Hacker.
1. Install the free Resource Hacker from
http://www.angusj.com/resourcehacker/.
2. Copy the file 'Classic Skin.skin' from C:\Program Files\Classic Shell\Skins\ to some location like the Desktop and rename it something else. Renaming is important so it doesn't get overwritten when Classic Shell is updated.
3. Open Resource Hacker and open the skin file in it.
4. Expand the Bitmap section and expand all the way till you see 1 -> 1033. You need to update bitmap 1.
5. Right click on 1033 resource below Bitmap -> 1 and save the bitmap 1 from Resource Hacker.
6. Then open it in some image editor which supports 32-bit BMPs with transparency such as Photoshop.
7. Change the portion you wish to update and make sure you save it as a 32-bit BMP and not 24-bit one.
8. Right click 1033 again and click "Replace resource" and replace it with the new bitmap.
9. Save the skin in Resource Hacker and copy the renamed file back to C:\Program Files\Classic Shell\Skins.
10. Open Start menu settings and from the Skin tab, switch to the new modded skin.