To customize your Product's appearance, go to your "Products."
Then, select which Product you'd like to customize, then click on "Appearance" in the sidebar.
If you're happy with your Product theme, but want to change the fine details (colors, text, imagery, etc.), click the "Customize" button on the Current Theme.
You'll then be brought to the Theme Editor. On, the left will be a sidebar labeled "Settings". This is where you'll make all your changes! (Note: Settings are different for each theme, so don't worry if they don't look exactly like this!). At the left of the footer is a Mobile / Desktop view switcher, so you can see what your Product looks like on a mobile device. At the right of the footer is a Current Page selector, allowing you to navigate the different pages in your Product (The Current Page selector will display the first instance of the selected page. For example, the Post selection will display the first Post in the first Category of your Product.)
You can change your Settings either by navigating through the Settings sidebar on the left, or by clicking on the specific element you wish to change.
This will automatically navigate you to the specific setting in the Settings sidebar.
Have a bunch of fun customizing your Product's appearance to your heart's content!