For Child Theme Users
There are two important changes.
First, the image and icons folders have to be in a subfolder called “core”. Please see it has been done for the example child theme.
Secondly, the media queries have been moved to a separate css file. So now you do not need to have the default media queries in the child theme any more.
Please see the rest of the changes in the changelog.
Here is the changelog on GitHub
Here is the diff from the trac