Steps of customizing an existing theme can be broken into three major steps

1. Select the base theme.
2. Create a sub-theme from the base theme
3. Make the changes to the new sub-theme

The relationship between a base theme and a sub-theme is almost like a parent-child relationship in object oriented programming. The child also called sub-theme which inherits its properties and characteristics from its parent also called the base-theme. A sub-theme can be the child of another sub-theme. There are no limits to chain together multiple parent-child relationships.

Never ever touch the original files provided with download, it is because when we upgrade them we won't be losing our customizations work.

List of Starter Themes is as under

1. AdaptiveTheme
2. beginning
3. beginningW2
4. BigDaddy
5. Boron
6. Clean
7. Flexible
8. Foundation
9. FuelDeLuxe
10. Genesis
11. Groundwork
12. Tao
13. Tendu
14. Yahoo Pure.CSS Framework
15. Zen

Read more about Starter Themes

Steps to Create a sub theme, taking bartik as parent theme

1. Copy the bartik entire theme folder and paste it in /sites/all/themes
2. Rename the bartik folder name to eg bond007
3. Rename the bartik.info file to bond007.info
4. Delete all the file under the templates folder except the page.tpl.php
5. Delete all the file under css folder
6. Replace the occurrence of `bartik` in file's inside the bond007 folder and with bond007

Sub-themes do not inherit any custom regions defined in parent theme. We specifically need to re-specify them in sub-theme, in our case we need to redefine the custom regions in bond007.info file. If we do not declare regions at all, we will get the following regions by default: header, help, highlight, content, sidebar first, sidebar second, and footer. and also we need at least one stylesheet. As this is a requirement for a valid sub-theme.

Thanks