If you are using the free Spark Starter plan, the Adobe Spark branded header and footer will appear on all Spark Pages. With the full version of Spark, you will have the ability to customize both the header and footer.
If you are using the full version of Spark and you would like to customize the header and footer of Spark Pages, first set up your brand by clicking "Personalize this design" at the top left of the "Your Projects" page. If your brand is already set up, click "Manage Brand". From there, scroll to the bottom and click "Page Header and Footer". Please note that there are various options to click at the top right of the page.
While on this screen, if you would like to add social icons to the footer, click "Social" to add links for Facebook, Twitter, and Instagram.