1. Enter Themes and Branding in the Quick Find box, then select Themes and Branding from Setup.
2. View, preview, and activate an existing theme, or click New Theme to create your own.Keep these considerations in mind when working with themes:Only one theme can be active at a time, and a theme applies to your entire org.By default, orgs use the built-in Lightning Blue theme.You can create up to 300 custom themes, but you can’t modify or clone the built-in themes provided by Salesforce. Themes don’t apply to Salesforce Classic or to mobile.Chatter External users see the built-in Lightning Blue theme only.
3. A Theme applies to your entire org but only one theme can be active at a time
4. Orgs use the built-in Lightning Blue theme by default.
5. You can create up to 300 custom themes, but you can’t modify or clone the built-in themes provided by Salesforce.
6. Themes do not apply to Salesforce Classic or to mobile.
7. Chatter External users see the built-in Lightning Blue theme only.