fix(index): use light theme as default fallback

index.html

@@ -34,6 +34,7 @@
       search: ['/'],
       darklightTheme: {
         siteFont : 'Source Sans Pro, Helvetica Neue',
+        defaultTheme : 'light',
         codeFontFamily : 'Roboto Mono, Monaco, courier, monospace',
         bodyFontSize: '15px',
       }