If you share your computer with many people, then there is possibility that everyone is installing their own themes on your computer. If you do not like this, you can disable changing of themes in Windows 7. Here is how :
- Press the key combination Windows Key + R to open the Run dialog. Type gpedit.msc in the Run dialog and press Enter.
- In the Group Policies Editor, navigate to User Configuration → Administrative Templates → Control Panel → Personalization in the left side treeview.
- In the right hand side pane, you will find a value Prevent changing theme. Double click on it. Choose Enabled and click OK to save the settings and close the dialog.
With this group policy setting enabled, no user on your computer will be able to change or save any theme.