Once you create you project and are in edit mode, drop a container onto the bottom of the page. Select the 'Style' tab in the top right and scroll down to the bottom to find the 'Advanced' section. You will be adding to CSS values here.
Fun Fact: If you are using a phone or tablet page type, you may need to wrap all your page content in one container. Like so:
If you have any questions, please contact us in Support.
How to Create a Floating Header.