Category class

what theme are you using?

can you post a link to your site?

your theme info says it is based on Twenty Ten;

possibly look for loop.php of your theme;

look for a line with <div class="post… and some more code;
edit this accordingly.

or possibly paste the full code of loop.php into a http://pastebin.com/ and post the link to it here.
someone might have look at it and might be able to make some useful suggestions.

and also try to contact who ever build that theme for you – this might be easier and faster.

close to the beginning of the code, see these lines:

<?php if ( in_category('3') ) { ?>
           <div class="post-cat-three">
 <?php } else { ?>
           <div class="post">
 <?php } ?>

define what you would like instead, and edit the code.

edit:
the style.css does not seem to have any styles for .post-cat-three – therefore you could simplyfy the above code:

<div class="post">

yes – 3 is the category ID of your category ‘inspiration’ or so.

to find the category IDs of your categories, goto
dashboard – posts – categories; and hover over the category names in the list, then look at the address bar at the bottom of the browser:
it will show ........&tag_ID=3&..... where 3 is the category ID of the category where the mouse pointer hovers over.

http://codex.wordpress.org/Function_Reference/in_category

<?php if ( in_category('3') ) { ?>
<div class="post-cat-three">
<?php  } elseif ( in_category('4') ) { ?>
<div class="post-cat-four">
<?php } else { ?>
<div class="post">
<?php } ?>

structure:

if
elseif
elseif /*repeated as often as needed*/
else

http://php.net/manual/en/control-structures.elseif.php