i build a simple blank theme what should i do to make changes in background colour and attachment of pic over there??

This is the AUI forum, your question probably belongs in the dev forum instead.

Background color and pic would both be defined within the theme's CSS.

Hi Akash,
Add a file named custom.css inside docroot/_diffs/css and the following portion of custom.css handles the background part.you should configure according to your requirement.
body {
/*background-color: #0066CC;*/
background: url("../images/xxx.png") no-repeat;

/*color: #FFFFFF;*/

font-family: Verdana,Helvetica,sans-serif;
font-size: 11px;
}
once you add the your own css in the file,it would override the docroot/css/custom.css.
Hope this helps...
Regards
Rojalin

Thank You So much ROjalin!!!