See Sightings for examples of how Sputnik can look.
Basics
The absolutely easiest way to customize Sputnik a little bit is to change some of the parameters in "sputnik/config". For example, you can change the background color ("BACKGROUND_COLOR"), the logo ("LOGO"), etc. See Configuration and More Parameters.
Styles
You can change how Sputnik looks by changing the stylesheets. See Styles.
Templates
For more drastic changes, you can edit the HTML templates. See Templates.
URLs
To configure Sputnik URLs, see Nicer URLs.
Internationalization
Sputnik's interface is internationalizeable. Three languages are supported by default and adding new transcriptions (or fixing the existing ones) is easy. See Internationalization.