Well, Boron doesn't actually comes with built-in RTL support (yet), but this HTML5 theme provides such a basic style sheet that making it work with RTL is just as simple as making it work with LTR.
Boron is a base theme for Drupal which converts the core template files to HTML5 markup. Like other minimal base themes, such as Zen and Stark, it includes only a few lines of layout-driven CSS, and markup that is search engine optimized.
Unlike the others, Boron utilizes the new HTML5 doctype and semantic elements, such as Article, Header, Footer, and Section. The result is a standards-compliant theme with valid markup and minimal CSS that can be used as a jumping off point for developing new sub-themes. This leaves your site positioned to take advantage of new HTML5 features as they come into vogue.