fix(css,preview): scope utilities under the view root, preview with real boards and foreign css

preview/index.html

@@ -6,6 +6,8 @@
     <link rel="stylesheet" href="/app.css" />
     <link rel="stylesheet" href="/theme.css" />
     <link rel="stylesheet" href="/styles.css" />
+    <!-- Other plugins' stylesheets (EXTRA_CSS), after ours like a later plugin. -->
+    <link rel="stylesheet" href="/extra.css" />
     <style>
       body.theme-dark,
       body.theme-light {