Depending on your coding comfort level, this GitHub site was HUGELY helpful in getting started for us: catalog/branding at master · unsupported/catalog · GitHub
For an idea of what we've done at GSU (Stacks is our branding for Catalog): Georgia State University
We modify the stylesheets to change the colours and presentation of elements on our Catalog.
To add on other features like Image Slider, Categories and Dividers, Extended Navigation Menu, we rely on JQuery to inject HTML code into the existing layout.
Here is how our Catalog looks like: Learn for Life eCampus