categories background

If by CELL, you mean some containing HTML block, like a <div> or <li>, you could possibly use a PHP if statement to give the blocks different CSS classes, e.g. <div class="some-cat">.
Then in you stylesheet set the background or whatever property, using these classes.
Such as:
.some-cat{
background-color: yellow;
}

Ok but how to set my index.php where the posts from the 3 different categoies appear?

Thank you

up

Allright! Maybe your answer can be found in this thread?
https://www.ads-software.com/support/topic/60671?replies=4#post-324444