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Windows XP Tips, Illustrations and registry edits
A desktop theme is a predefined set of icons, fonts, colors, sounds, and other window elements that give your desktop a unified and distinctive look. In the Themes dropdown list, you may see "More themes online" entry which takes you to the Microsoft Plus! Home page. Similarly you can add your own Web locations / Third-party Themes sites and list it in the Themes drop-down box.
Here is how:
HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows \ CurrentVersion \ Themes \ DownloadSites
HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows \ CurrentVersion \ Themes \ DownloadSites \ 101
In the Themes drop-down, you'll see a new entry named Third-party themes which leads you to the above website when selected. Similarly, repeat the steps and create as many entries as you want.