You can create custom stylesheets, or "skins", for your works just like you can create skins for the Archive. The main difference is that work skins will change the way your work appears to other users, not just you.
Work skins will only affect the body of the work they are applied to—you can't change the Archive navigation or background with them. What you can do, however, is create your own classes. For instance, you can change the color of some of your text, indent some paragraphs in a particular way, and so on.
For example, let's say that you want to make a single word in your text bright blue. Here is what you might do:
.bluetext {color: blue;}
I
want <span class="bluetext">house</span> to be in blue
Refer to Tutorial: Creating a Work Skin and Skins and Archive Interface FAQ for more information.