Files affected after Magento upgrade

  - Your magento's default theme is replaced by the new default theme after upgrading your Magento store.
 So, it is not a good idea to edit the default theme. If you want a new theme based on its default theme,
 you need to duplicate the default theme and give a new name. Please refer to "Adding new magento theme".

Adding New Magento Theme

  1. You can download free magento themes from http://www.templates.com/blog/free-magento-themes-for-your-web-store/,
  create your own theme or create a theme based on the default theme.
2. Place the design files of your theme in magento_root_directory/skin/frontend/default/[template_name] and
  the code files of your theme in magento_root_directory/app/design/frontend/default/[template_name].
3. Login to magento admin panel and go to System -> Design
4. Click "Add Design Change"
5. From the "Custom Design" drop down list, select your newly created theme.
6. You can also specify the date range for up to when a theme will display by filling-out "Date From" and "Date To" fields.
7. Click "Save"
  You can add more themes for as long as all theme will have different date range.