Try to play with your templates/tmpl_uni/css/general.css file ;-)

For example:

body
{
...
background-color:transparent;
(change/comment/delete background-image) ...
}

Also play with .main class...
Search for tags regarding main block, block header, block content, etc... in the same general.css file
Especially for the block, you can also try to replace the default background image with semi-transparent image ;-)

If you need more info, try a search on Google with "css see more background" as keywords ;-)