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.
SFDC Lessons 
Arun, how do you apply a Theme to just a single App & Profile?
– for example, I have two apps
– need a blue background for App1 (Profile = Sales)
– need a red background for App2 (Profile = Carrier
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