Add basic background color generator

Supports specifying background colors as an array of colors, or an
object where the label is what you want to name that color as a
background utility, and the value is the color to use.

Everything can be specified as either camelCase or kebab-case so you
don't need to quote the shit out of everything but classes will always
be rendered as kebab-case.

docs/source/_assets/less/main.less

@@ -5,8 +5,13 @@
     @class "py-2";
     background-color: green;
   }
+  @media (--breakpoint-md) {
+    background-color: orange;
+  }
 }
 
+@tailwind utilities;
+
 .bg-light {
   background: #fff;
 }