[DOCS] Adjust watermark color
[DOCS] Adjust watermark color

The default watermark color is distracting and makes the content
difficult to read. Tone it down to a light gray.

Change-Id: I2ca547dca93f89edb1416ac44169749028037621

weezer/openstack-ansible,robb-romans/openstack-ansible,yanyao/openstack-ansible,bunchc/openstack-ansible,BjoernT/os-ansible-deployment,cloudnull/os-ansible-deployment,yanyao/openstack-ansible,Logan2211/openstack-ansible,Logan2211/openstack-ansible,os-cloud/os-ansible-deployment,cloudnull/os-ansible-deployment,robb-romans/openstack-ansible,bunchc/openstack-ansible,weezer/openstack-ansible,openstack/openstack-ansible,openstack/openstack-ansible,os-cloud/os-ansible-deployment,BjoernT/os-ansible-deployment,cfarquhar/openstack-ansible,BjoernT/openstack-ansible,cfarquhar/openstack-ansible,BjoernT/openstack-ansible,stackforge/os-ansible-deployment,BjoernT/os-ansible-deployment,stackforge/os-ansible-deployment