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Initial stable release of 7.x-3.x
This is a major upgrade of the base theme will almost every line of code rewritten and many new features for site builders, theme developers and those hard core PHP developers out there using this framework.
Please see the Project page, there is a brief run down there of the main features and changes, also check the issue queue, there are a couple of exisiting issues (mainly tasks) but I envision there will be some bugs and other issues that will crop up in the next weeks or so prior to a full stable release.
None of the existing sub-themes have been update to work with this, but for the most part should work, see below.
Notes About Existing Subthemes:
Your 7.x-2.x sub-themes * should* continue to work as per normal, but I can't guarantee this. There are two main issues you could run into - the gutters could look different, I need to document how to fix this, its not major, quite literally channging a couple of class names in the CSS, the second is more fundemental on the theme settings page, you could run into an "illigal choice" error which will almost certainly require you to update some lines of code in the info file - in essence you need to change the responsive layout defaults from using hyphens to underscores, for example in the info file change 'three-col-grail' to 'three_col_grail'. Again I will document this in an issue.