In recent days we've solidified the direction that themes will be going in Moodle 2.7 so I wanted to make sure everyone was aware of these things nice and early:
- Clean is the new default theme for Moodle. This theme is not much changed from 2.6 and is basically our Bootstrap2 base with a couple of basic settings to change the logo etc. It's fast and is what developers will be testing all code changes with. It also functions as a nice example of a simple theme that you can clone and use to build new themes. If you use the .less files then you will need to use your own tools to generate css from them.
- We have a new Bootstrap2-based theme called More that contains a lot more settings for customising the theme from the UI. The goal for this theme is to provide a way for any admin, even very inexperienced, some way to quickly customise a Moodle site just by changing colour settings etc. Over time I hope it gains more abilities with your input. In the background is some fancy technology to compile .css files automatically from .less files. We do not recommend that you copy this for all your themes, and our feedback is that most designers are not interested in this.
- All our old Standard-based themes are being removed from core and will be moved to github (for anyone who wants to still work on them) and will be provided in Moodle plugins for download by anyone who still wants them. For more see MDL-43784.
Finally, we have no current plans to put Bootstrap 3 in core Moodle for the time being (discussion here: MDL-40177). Our goal is to reach good stability for themers and chasing after the latest releases from Bootstrap will just break themes (and doesn't seem to give us many advantages either). This doesn't stop you using Bootstrap 3 in your own themes if you really want to, as many already have.