Since you have the Windows 7 Starter
edition, you options for themes is very limited since that edition does not include the "Personlization" feature in it.
Compare Windows 7 Editions
OPTION ONE in the tutorial below can help show you how to change a theme in Windows 7 Starter.
Theme - Change or Save
However, you might also consider using the free program Personalization Panel
from OPTION TWO in the tutorial below to add a few more options.
Hope this helps,