A new 140plus feature lets you select different themes by hashtag (category). You can use one theme as the main theme for your site and different themes for specific hashtags.
For example, as shown, the mrblog.140plus.com site uses the Journalist theme as the main theme but uses the Facepress photojournal theme for the #arts hashtag category:
Main blog site: http://mrblog.140plus.com
#arts category: http://mrblog.140plus.com/category/arts