Depending on the contract you signed you may have the ability to get Bynder creative services to support some limited changes to support brand look and feel, however these typically tend to be more about what fonts are used, custom header and navigation, custom styling for homepage elements and not the main DAM or Collection pages itself.
There is now a lot of self service management for basic use of fonts, colors etc, what fields that display on tiles etc in the configuration settings under Advanced Settings > Portal Settings > Look and Feel > Portal Theme: https://support.bynder.com/hc/en-us/articles/360013936259-Customize-Portal-Look-and-Feel