Update theme-gray-600 and gl-text-color-tertiary
Update theme-gray-600 and gl-text-color-tertiary

mmkassem/gitlabhq,jirutka/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,stoplightio/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,iiet/iiet-git,dreampet/gitlab,mmkassem/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,axilleas/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,iiet/iiet-git,iiet/iiet-git,dreampet/gitlab,dreampet/gitlab