First, you navigate into the settings by clicking the gear icon on the top-left side.
On the side bar navigation, locate the Marketing and click Themes.
You will be redirected to the list of the themes available inside your portal.
Select the theme that you wish to update by hovering the theme and clicking the View theme button.
The next window will be the settings available in your theme.
After you update the settings, click the Apply changes button and make sure that you publish any changes to implement your updates across the pages that are under this theme.
Reference video: https://share.vidyard.com/watch/wySLeJDkpxCUKsns94DHPF?