You can customize your Store Theme by giving your favorite colors to the Header, Footer, Background and Menu of the Store Theme.
Click on the respective color codes for each section, select a color, and see the changes being reflected instantly in your Theme.
Some Themes allow you to upload a cover image too for your Store.
Remember the Cover Image's width should be more than 1000px.
Once you are satisfied with the settings, click on 'Confirm' button to save your Theme Settings.