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    @AngeloRocha
I did manage to have a background on the regular blocks, my problem is that modules its self have a transparent background, example module xfguestbook have transparent background, because i have a background image its not display correctly 
this is my curretnt css for blocks
 /* 3. XOOPS Blocks*/ 
div.xoops-side-blocks > aside{background:#FFF; padding:10px; text-align:justify; margin-bottom:20px; border-radius:5px; -moz-border-radius:5px; -webkit-border-radius:5px; position:relative; transition:all linear 300ms; -moz-transition:all linear 300ms; -webkit-transition:all linear 300ms; top:0;} 
div.xoops-side-blocks > aside h1, div.xoops-side-blocks > aside h2, div.xoops-side-blocks > aside h3, div.xoops-side-blocks > aside h4, div.xoops-side-blocks > aside h5, div.xoops-side-blocks > aside h6{font-size:0.9em;} 
div.xoops-side-blocks > aside > h4.block-title{line-height:inherit; margin:0; font-size:1.3em;} 
 
div.xoops-blocks{background:#FFF; padding:10px; text-align:justify; margin-bottom:20px; border-radius:5px; -moz-border-radius:5px; -webkit-border-radius:5px; position:relative; transition:all linear 300ms; -moz-transition:all linear 300ms; -webkit-transition:all linear 300ms; top:0;} 
div.xoops-blocks > h4{line-height:inherit; margin:0;} 
 
div.xoops-bottom-blocks{background:#FFF; padding:10px; text-align:justify; margin-bottom:20px; border-radius:5px; -moz-border-radius:5px; -webkit-border-radius:5px; position:relative; transition:all linear 300ms; -moz-transition:all linear 300ms; -webkit-transition:all linear 300ms; top:0;} 
div.xoops-bottom-blocks:hover{top:-10px; box-shadow:0 10px 10px #222;}  
EDIT:
I have played with this:
 div.maincontainer{background:#FFF; padding:10px; text-align:justify; margin-bottom:20px; border-radius:5px; -moz-border-radius:5px; -webkit-border-radius:5px; position:relative; transition:all linear 300ms; -moz-transition:all linear 300ms; -webkit-transition:all linear 300ms; top:0;}  
I does make a white background on the modules, but it is now one big white block, this i not what i want, i want to separate them for a nice look