Reviewing through your inquiry I can confirm that this type of change would be possible in a Jive 8 hosted/on-prem instance through the use of an advanced custom theme in your instance. However, this type of feature is not available for Jive Cloud.
That being said, if you are interested in adding global custom css to your Jive Cloud instance I would suggest engaging with our Professional Service's team as they can work with you to determine the available options and assist in implementing them into your instance. To start a PS engagement either reach out to your account manager, or submit a case in your customer group here in the Jive Community and a member of our support team can get one set up for you.
Very helpful, now I don't have to search around ways to see if I am missing something in cloud, I know what my options are