The CLARIN Human Interface Guidelines were developed in the context of CLARIN-PLUS.
The actual guidelines document can be found among the CLARIN-PLUS deliverables as D3.1.
Additional information can be found on the Branding page of the CLARIN Trac.
A Bootstrap/LESS style based on these guidelines has been implemented and is maintained at and available from https://github.com/clarin-eric/base_style.