I use to add themes by uploading the theme to the themes folder and in order to remove i use to delete the folder <themename>

But now i realised that if i have used some themename earlier which i have deleted, then i cannot be use the samename again. as when i give the same themename the old design comes up. does XOOPS stores the themes code in db ??? i didnt see any such table ... although templates were there

Is it the cache problem or there is some other way of adding and removing themes.

Before you remove your theme, go to the admin>>preferences and select a new theme to use. Then fo to your <xoops_root_/themes folder and delete it if you want. If you delete a theme before changing the default them in your preferences, your site may not load properly. If this is the case, simple re-upload the deleted theme then follow the steps outlined above.

Hope this helps.