Existing Themes
There a handful of themes available for Backdrop, with more being added each month. There are several resources to find a theme for your site:
The Backdrop themes page has stable releases.
The official distribution channel for themes is the Backdrop themes page on backdropcms.org. This page lists all themes that have an official release (or pre-release). To find ports of Drupal 7 themes, search for the name of the theme.
The Github backdrop-contrib pepository contains all projects.
You can find all of the above, plus other modules, themes, and layout templates that are still in development in the Backdrop Contrib Github repository. Projects that haven’t been released yet are likely not ready for production use. If you find something that you would like to use, please test it in a safe enviornment (like one of our demo sandbox sites). If it looks ready to use, please drop a note to the developer expressing your interest (or possibly your willingness to help bring it to production).
Developing Themes
If you can’t find a theme that suites your needs, there is alwyas the option of creating one yourself — whether porting an existing Drupal 7 theme to Backdrop, or creating a new theme.
Porting an existing Drupal 7 theme
Documentation for converting Drupal 7 themes can be found here. Before starting a conversion, you should check the Drupal 7 project issue queue to see if a Backdrop port has been requested or started. You can do this by filtering the project's issue queue for the tag “backdrop-port”. If you don’t see an issue, (and there is not already a port in the Github backdrop-contrib repository), then it’s probably safe to start the port.
Creating a new theme
You may also be able create a custom theme with some CSS and HML knowledge. Documentation on developing new themes can be found here.