The basic templates are defined in sputnik/templates. Nodes can be configured to use their own templates. (Set the "templates" parameter.) Those can override specific chunks of UI, or switch your whole site to use a different set of templates. (See LuaUsers demo for an example.)
Sputnik uses Cosmo as it's template library. (Cosmo used to be a Sputnik module but has since got its own life.)