The Import Brand button sits above the screens on the Design tab, next to Reset. It's there on every screen, so you don't need to switch to Branding first.
How to Import Your Brand
Open Import Brand
Enter your domain

Review the changes
Apply

What Gets Imported
Brand Colors
Chat icon background, header background, button background, user and assistant message backgrounds, and the send button
Logo
Your company logo for the lead form header
Bot Avatar
The image shown beside your chatbot’s messages
Header Background
A background image for the lead form header