Grabbed dndbeyond's source code

```
~/go/bin/sourcemapper -output ddb -jsurl https://media.dndbeyond.com/character-app/static/js/main.90aa78c5.js
```

ddb_main/node_modules/jss-plugin-rule-value-observable/dist/jss-plugin-rule-value-observable.esm.js

@@ -0,0 +1,49 @@
+import $$observable from 'symbol-observable';
+import { createRule } from 'jss';
+
+var isObservable = function isObservable(value) {
+  return value && value[$$observable] && value === value[$$observable]();
+};
+
+var observablePlugin = function observablePlugin(updateOptions) {
+  return {
+    onCreateRule: function onCreateRule(name, decl, options) {
+      if (!isObservable(decl)) return null;
+      var style$ = decl;
+      var rule = createRule(name, {}, options); // TODO
+      // Call `stream.subscribe()` returns a subscription, which should be explicitly
+      // unsubscribed from when we know this sheet is no longer needed.
+
+      style$.subscribe(function (style) {
+        for (var prop in style) {
+          rule.prop(prop, style[prop], updateOptions);
+        }
+      });
+      return rule;
+    },
+    onProcessRule: function onProcessRule(rule) {
+      if (rule && rule.type !== 'style') return;
+      var styleRule = rule;
+      var style = styleRule.style;
+
+      var _loop = function _loop(prop) {
+        var value = style[prop];
+        if (!isObservable(value)) return "continue";
+        delete style[prop];
+        value.subscribe({
+          next: function next(nextValue) {
+            styleRule.prop(prop, nextValue, updateOptions);
+          }
+        });
+      };
+
+      for (var prop in style) {
+        var _ret = _loop(prop);
+
+        if (_ret === "continue") continue;
+      }
+    }
+  };
+};
+
+export default observablePlugin;