Front-end developers takes the visual designer’s work and turns it into a web page or interface. They program interaction with JavaScript and/or a library such as jQuery, YUI or Prototype. They test cross-browser, cross-platform, and/or cross-device compatibility for inconsistencies.
Themers are front-end developers with specialist knowledge of the theme layer in Drupal. They create sub-themes on common base themes and may create custom themes. This might not be a distinct role on many teams. Instead, a developer on the team might have this expertise.