The css used to modify the description within the clients listing details page by default uses the css from your site at large. This can be a problem with some css in that what you set for a specific html tag (h1, ul, li,…etc) for your site may not be good to display the within the clients description. Since you may allow your clients wide use of different formatting html within the following admin tool:

SITE SETUP > ALLOWED HTML

and as an option directly within the wysiwyg for the clients description when placing a listing you may find it necessary to just setup css that will only modify your clients descriptions and provide a more "expected" result when they choose to use specific html tags you allow them.

To setup specific css for the description you would need to surround the "description scope" with it's own label within the html tag surrounding it and them create those specific css within your css file. So you would edit the following templates that control the display of the listing within the listing details page:

listing_classified.tpl
listing_auction.tpl

You would find the tag that places the description within that template:

{$description}

and then place a div tag with your custom css label to it like so:

<div id="client-description">{$description}</div>

and then edit your custom.css file to include the specific css you want to modify for the client description like so:

#client-description h1 {  };
#client-description ul {  };
#client-description li {  };

…etc

You would only need to add the html tags to the above that you actually allow within the ALLOWED HTML admin tool and those that you want to use css specific to the description and not for your site in total.