feat: add more example fixtures for testing

Author: phoenisx

examples/basics/local.css

@@ -14,4 +14,5 @@ body {
 .container {
   width: var(--container-size);
   padding: var(--container-padding);
+  background-color: var(--test-custom-selector);
 }

examples/basics/theme.css

@@ -13,3 +13,15 @@
     --container-padding: 1rem;
   }
 }
+
+/* CSS with new at-rules */
+
+@custom-selector :--root :root, :after, :before;
+
+@layer variables {
+  @layer colors {
+    :--root {
+      --test-custom-selector: rgba(17, 17, 17, 1);
+    }
+  }
+}