MB-10645: improved warm up section style

Change-Id: If0b2fc04b3bbf86422287f3ea45ec2ea8fa78a2c
Reviewed-on: http://review.couchbase.org/39407
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>