It is very easy to create custom theme for share point sites. A SharePoint site theme basically consists of three kinds of files:-
1. theme.inf
2. theme.css
3. image files.
Just follow the following steps and you will get your custom theme. Suppose your theme name is "myTheme".
1. Go to the folder "C:Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\THEMES" folder. Every Folder under this Folder defines a theme. Just make a copy of any of the the Folders and name it "myTheme". Suppose you copied BELLTOWN folder.
2. Rename BELLTOWN .INF to MYTHEME.INF. Keep in mind the uppercase thing.
3. Open MYTHEME.INF file with notepad and replace all BELLTOWN to myTheme.
4. Open "C:Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATELAYOUTS\1033\SPTHEMES.XML" file with notepad.5. Add the following lines under
<>
<>
<>
<>images/
<>images/
< /templates >
6. Create a picture to describe your theme and store that in "C:Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\IMAGES" folder with myTheme.gif name.
7. Restart IIS.
8. Now go to Site Settings, now choose myTheme theme.
9. Now to edit this theme open SharePoint Designer 2007. Go to "File" -> "Open Site…" and type the URL of your site.
10. In the Left menu you will find a lot of Folder,find a folder named myTheme which is your custom theme.
11. Find myTh-65001.css file in this folder. This is the CSS file for your theme. Edit this according to your need.
12. There is another css file named CORE.CSS in "C:Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUT\1033\STYLES" which contains styles for all sites. You may need to override some css attributes from this file.