Added if check to see if user is using custom CSS

Author: HrBingR

server/layout.js

@@ -40,11 +40,16 @@ module.exports = function(state, body = '') {
           type="text/css"
           href="${assets.get('app.css')}"
         />
-        <link
-          rel="stylesheet"
-          type="text/css"
-          href="${state.ui.assets.custom_css}"
-		/>
+        <script>
+        var custom_css_file = '${state.ui.assets.custom_css}';
+          if (custom_css_file!="undefined") {
+            var custom_css_enabled = document.createElement('link');
+            custom_css_enabled.rel = 'stylesheet';
+            custom_css_enabled.type = 'text/css';
+            custom_css_enabled.href = '${state.ui.assets.custom_css}';
+            document.head.appendChild(custom_css_enabled);
+          }
+        </script>
         <link
           rel="apple-touch-icon"
           sizes="180x180"