Summary
Although the default XHTML output from the DITA OT is quite plain, there are many ways to customize its appearance so that it looks the way you want it. The most dramatic changes can be achieved by modifying the CSS. Creating custom headers and footers adds a note of consistency to your documents.
Modifying the XSL transforms can add dynamic changes that are driven by many different external and internal influences. To make your DITA OT changes much more portable, create a DITA plugin. Specializations help you to make your topics much more content specific.
Now, get in there and hack!