Support standard css variables, dynamically set spacing-unit

src/core/client/ui/theme/global.css

@@ -0,0 +1,5 @@
+@media (max-width: $breakpoints-sm) {
+  :root {
+    --spacing-unit: calc(0.5 * var(--spacing-unit));
+  }
+}

src/core/client/ui/theme/variables.ts

@@ -58,7 +58,7 @@ const variables = {
     },
   },
   /* gitter and spacing */
-  spacingUnit: 5,
+  spacingUnit: 10,
   /* Borders */
   roundCorners: "2px",
   /* Typography */