Add theme support custom header

I’m using the Liquorice theme. The theme doesn’t come with a custom header option in admin but I was hoping to add it using add theme support. I added the code as instructed here to the functions.php file and now have the Customer Header option available to me under Appearance. I go through the steps there to upload the new header image I want to use (I named it default.jpg, maybe that’s not the right thing to do.) and it says “Header updated. Visit your site to see how it looks.” but when I look at the site it is unchanged. Do I need to change something somewhere else? Maybe in the stylesheet? Did I mess up the code I was supposed to add to the functions.php file? I don’t know what I’m doing wrong. Any help is greatly appreciated!