Customizing your own topic is pretty easy. After your first create your topic, go back and edit it. Make sure you are editing the topic and not the post!
Expand the "admin" section by clicking on the plus sign. One of the fields that will reveal itself is called "Include in Head Tag". This is the text box that you will want to type in.
Instead of going into a detailed example of how CSS works, I will just give you an example. Cut and paste the following code, but replace the brackets around the style words with lesser and greater than signs.
You can change the picture by replacing the address inside of the url() container.
[style]
body{background-image:url(http://www.animecutie.com/gg_data/images/0/creey-yet-awesome-3976.jpg);}
.mcmain{background-image:none;}
.google{height:0px;visibility:hidden;}
.mctop{background-image:none;visibility:hidden;height:0px;}
.mccontent{background-image:none;}
.mcbackground{background-image:none;}
.mcbottom{background-image:none;height:0px;}
[/style]
You can examine one of our role playing topics
http://www.animecutie.com/topic-blood-of-the-demon-1094.html to see how the above code changed it's appearance.