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Themes/Dark Mode is often the most procrastinated feature of implementing dark mode in your app’s front-end. Write a CSS mixin containing classes of DOM elements we want to style as a part of our theme. Write your code to allow the user to change the class of the <body> element. Use the same setTheme method for the onChange event that already sets the body class and stores the user's. The setTheme is set on a drop-down to select from multiple themes.