It's Cascading Style Sheets, a stylesheet language used to modify the presentation of something written in HTML/XML.
I don't know what works in here, but you should be able to customize layout, colors, fonts... That kind of thing.
A sample code would be this:
CSS:
color: #ff0000;
background-color: #f0f0f0;
border: 2px solid #000000;
font-size: 14px;
padding: 10px;
You might be able to target specific elements (if it indeed follows the HTML structure), like this:
CSS:
.username {
font-weight: bold;
color: #0066cc;
}
There's also some bullshit like visual effects, would go something like this:
CSS:
color: #333333;
background-color: #f9f9f9;
border: 1px solid #cccccc;
padding: 10px;
box-shadow: 2px 2px 8px #aaaaaa;
text-shadow: 1px 1px 3px #bbbbbb;
transition: background-color 0.3s, box-shadow 0.3s;
:hover {
background-color: #e0e0e0;
box-shadow: 4px 4px 10px #888888;
}
Just don't overdo it lol
Edit: Look up hexcode for colors