Update Your Community Look and Feel Quickly by Changing Themes
Admins and designers can now select from a library of pre-built themes and customize
them with clicks, not code, using new controls in Community Builder.
Where: This feature is available in Lightning communities, accessed through Lightning Experience and Salesforce Classic. Communities are available in Enterprise, Performance, Unlimited, and Developer editions.
How: To choose a new theme select, Change Theme in the Themes menu of Community Builder. Adjust the theme settings and components to meet the needs of your community and customer. Experiment to see what works best.