Update border-radius css and remove mixin

Change-Id: I4b7fbbb3ff260d84c5a622785d4f2930d0cc249d
Closes-Bug: #1369761
Bringing HACKING.rst in correspondence with tox.ini

Since there is a couple of new import exceptions in tox.ini,
we should add corresponding lines to HACKING.rst list.

Change-Id: Ia45eb9a5dcdc4db5205f235c85cfeab80c62d786
Merge "Update border-radius css and remove mixin"
Merge "Bringing HACKING.rst in correspondence with tox.ini"