alt131 - 8:18 pm on Feb 4, 2012 (gmt 0)
Hi Genesis99, and welcome to css :)
I don't want to interrupt you and penders, but a couple of thoughts that might help. First, in addition to all penders suggestions, consider installing firebug for firefox. That will help you identify the location (which file) of the styles being applied to each element. Second, a"gotcha" for new members at WebmasterWorld is that posts appear in the order of posting and are not grouped according to which specific post is being responded to. So, as penders mentioned, really helps to provide a little more information that working/not working.
Finally, check your expectations -
As penders has explained, you won't see that in your original styles because the <p>'s are blue so you will only see the red div background in the margin area of the <p>. As penders has suggested, clear the browser cache (set your test browsers to "check each page load"), validate the css files to check for errors, and (this makes no sense, I know) make a new file - wouldn't be the first time some invisible oddity caused a css file to fail.
I expect to see a div in red.