As mentioned in https://github.com/system-ui/theme-ui/pull/235#discussion_r308298513 it would be nice to have a way to set color modes via a css class. ```js colors: { default: { text: '#000', background: '#fff', primary: '#07c', secondary: '#05a', accent: '#609', muted: '#f6f6f6', }, dark: { ... }, extraDark: { ... }, pink: { ... } }, ``` ```html <body> <body class="dark"> <body class="pink"> <body class="extra-dark"> ```