fix: add getExtra method

Author: gaetan-hexadog

src/Theme.php

@@ -117,6 +117,14 @@ public function setExtra(array $extra): self
         return $this;
     }
 
+    /**
+     * Get extra data.
+     */
+    public function getExtra(): array
+    {
+        return $this->extra;
+    }
+
     /**
      * Set theme version.
      */