Well, that is a very interesting question?? You can find some satisfying answer from System Administrator guide.
For the time being you can try this using the already existing themes. Let’s consider for an example we want to modify sage green theme and then use it.
Here are some simplified steps designed with the help of the system administrator guide.
1. Go to “C:Program FilesSageCRMCRM62WWWRootThemes”
2. Make copy of original sage_green.css and then rename the Copied file as “CustomTheme.css”.
3. Here there are be 4 folders named “FCKeditor”, “Img”, “Reports” and “XSL”. Inside each of these folder copy the sage_green folder and rename it to new name i.e. CustomTheme.
4. Then you can make the colour changes by modifying “CustomTheme.css” file for e.g replacing the color code Dark Green (#006633) to Dark Blue (#3333FF), Mid Green (#669966) to Mid Blue (#3399CC) and Light Green (#99CC99) to Light Blue (#66CCCC).
5. If you want to use new images then you have to create folder with the above name with images under “Img”.
6. Navigate to “C:Program FilesSageCRMCRM62WWWRootThemesReportsCustomTheme” and make changes only in 2 files named “StdGrids.css” and “StdPlain.css” by replacing the dark shade (for example, #006633 from the Sage Green theme) with the new shade for your theme.
7. Navigate to “C:Program FilesSageCRMCRM62WWWRootThemesXSLCustomTheme” and edit the BASICHTML.XSL by replacing the dark and mid shades (for example, #006633 and #669966 from the Sage Green theme) with the new shade for your theme.
8. Log Off and do iisreset.
Make the theme available in CRM as follows
1. Go to Administration –> Customization –> Translations –> New.
2. Enter a Caption Code as new theme, for example, CustomTheme, Caption Family Type as Tags and Caption Family as CssThemes and then give your theme a nice name in translations. Then click on “Save” button.
3. Clear your cache . For example, in IE 7.0 go to Tools | Delete Browsing History | Temporary Internet Files | Delete files. The new theme can be selected from Administration | System | System Behavior and
from My CRM | Preferences.