<html class="theme-black">
 <head>
 </head>
 <body>
   <div class="root">
     Black theme
   </div>
 </body>
</html>
.theme-black body {
    --background: black;
    --color: white;
}


.root {
    background: var(--background);
    color: var(--color);
    padding: 30px;
}

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