MythTV has a variety of official and third party theme packages that allow you to customize your system to fit your current mood, or just to match better with the aspect ratio of your 16:9 TV.
MythTV allows for two types of theme, so you can control both the overall look and feel of MythTV as well as the on screen display (OSD) that is overlayed on the screen when watching TV or videos.
Though many of the third party themes are actually designed by active MythTV developers, the designers often request that we withhold the "official" designation until a theme is deemed 100% complete, including both 4:3 and 16:9 versions. Don't be surprised if some of the better themes are actually listed as third party themes. We strongly encourage you to check out all of the available themes and choose the ones that look best to you.
For a complete list of current themes, please see the MythTV Wiki entry for Themes