Customising a sharepoint site is really a challenging job and there may be situations in which you might need to create a new theme. The simplest way is to create the new theme by taking a copy of the existng them. SharePoint theme folder has mainly 3 elements theme.inf, theme.css and images.

1. Create a copy of any one of the theme folder in “C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\THEMES” and rename as “mytheme“. For example copy the folder named “SAMPLE” and rename it to mytheme.
2. Rename the SAMPLE.INF file to MYTHEME.INF, remember all letters should be in upper case.
3. Open MYTHEME.INF file in your favourite text editor and change the value of title as per your needs.
4. Replace every word “SAMPLE” with “MYTHEME“
5. Change theme.css file to include your custom styles
6. Save and close the editor
7. Then open C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\1033\SPTHEMES.XML in a text editor
8. Add the follwing  lines under <spthemes> tag

<templates><templateID>Mytheme</templateID>
<displayName>Mytheme</displayName>
<description>My new custom theme.</description>
<thumbnail>images/home.gif</thumbnail>
<preview>images/home.gif</preview>
</templates>

9. paste your home.gif in images folder.
10. do IIS reset.

How to test your new theme

Got to site actions–>site setting –> look and feel–> you can see your custome theme there