Brand your Salesforce org in Lightning Experience.
We can customize themes & brands in the Salesforce org with the “Themes and Branding” features. We can use salesforce built-in themes or we can create our own custom themes with just a few clicks.
Go To | Setup | Search ‘theme’ in the quick search box, then select “Themes and Branding”.
We can preview/activate the salesforce built-in themes. OR we can create our own custom theme.