Storage Modules
Sputnik defaults to using a file system to store the wiki pages. If you want, however, you can store your data in a different form: it's just a matter of telling Sputnik which "storage module" you want to use. For a list of available modules and how to use them, see Using Storage Modules.
Markup Format
Sputnik defaults to Markdown as its markup format, but can also be used with POD.
Interface Language
Sputnik defaults to an English interface, but it is easy to switch between several languages. If your language isn't supported, it is trivial to add it. See Internationalization.
Look and Feel
For a simple change of colors, edit _colors. See Color Schemes for more information on how to do this more effectively.
For a more substantial redesign, edit _templates (or specify a different templates page). See Cosmo for information on the Sputnik's template format.
Advanced Changes
If you want to consider more radical changes, read Conceptual Model and see if you can achieve what you need with custom Actions. If this turns out to not be enough, see the Architecture section.