There are a number of library settings that can be used by local system administrators to customize the public catalogue.
See the section called “Library Settings Editor” for additional information on editing library settings.
Sitka Training Video - Adding Messags to Your Public Catalogue (4:26)
Messaging on the public catalogue can be customized with a temporary banner with text and links which is applied across the top of your public catalogue, and a custom message displayed on log in.

The banner has four display options which change the text colour:
Coop support recommends using "danger" for the best contrast.