I would suggest you please add this custom CSS code to the global CSS.
background-color: rgb(113,196,203) !important;
background-color: rgb(21,23,74) !important;
The purpose of providing custom CSS is to show you how to add CSS code to your site. Writing custom CSS is outside the scope of our theme support. If you need more customization, you need to learn CSS and learn how to use the browser’s element inspector.In case you have no idea about coding you can subscribe to One where customization questions are answered.
Hope it helps.