You can create or restore your own theme at any time by simply right clicking on the desktop and clicking the theme drop down menu. That will be where to look any available themes. You should "My current theme" followed by the XP theme and then the classic Windows theme.
At any time you can also save a theme like "My default theme" and click on the "SAVE" button to save it in MyDocuments or another folder. If modified theme is later created you can still reload the default theme you saved by browsing to it. loading it, and clicking the apply button.