I've been using Garland for a long time, but our staff complain that it's drab. Anyone have a suggestion for something brighter but also very clean (ie. good workflow, low rendering overhead)?
Personally I often like https://www.drupal.org/project/liquid_coolness
Simple, clean theme, that is still pleasing to see. Has liquid width, so if you have a big screen you can make use of it for CiviCRM screens.
Not really maintained though.
I've always used Adminimal. I think it makes the Drupal backend as a whole much easier to use, and it's always worked well for me with the Civi backend too.
There is also a CiviBartik theme that I use often: https://www.drupal.org/project/civi_bartik
A couple of others not listed in other answers
I use Bluemarine, which is an old classic: https://www.drupal.org/project/bluemarine
Not very exciting, but efficient use of screen space if you disable sidebars
I've been using a couple of themes based on the AdaptiveTheme https://www.drupal.org/project/adaptivetheme. I find both Pixture Reloaded and Corolla to be easy to use and adapt for large screens.