support concept of default theme

config.sample.json

@@ -7,6 +7,7 @@
     "bug_report_endpoint_url": "https://riot.im/bugreports/submit",
     "enableLabs": true,
     "default_federate": true,
+    "default_theme": "light",
     "roomDirectory": {
         "servers": [
             "matrix.org"

src/vector/index.html

@@ -25,9 +25,8 @@
         var match = file.match(/^bundles\/.*?\/theme-(.*)\.css$/);
         if (match) {
             var title = match[1].charAt(0).toUpperCase() + match[1].slice(1);
-            var light = match[1] == 'light';
         %>
-           <link rel="<%= light ? '' : 'alternate ' %>stylesheet" title="<%= title %>"
+           <link rel="alternate stylesheet" title="<%= title %>"
                href="<%= file %>">
         <% } else { %>
            <link rel="stylesheet" href="<%= file %>">

src/vector/index.js

@@ -281,6 +281,23 @@ async function loadApp() {
         configError = e;
     }
 
+    // as quickly as we possibly can, set a default theme...
+    const styleElements = Object.create(null);
+    let a;
+    for (let i = 0; (a = document.getElementsByTagName("link")[i]); i++) {
+        const href = a.getAttribute("href");
+        // shouldn't we be using the 'title' tag rather than the href?
+        const match = href.match(/^bundles\/.*\/theme-(.*)\.css$/);
+        if (match) {
+            if (match[1] === (configJson.default_theme || 'light')) {
+                // remove the alternative flag off the stylesheet
+                a.setAttribute("rel", "stylesheet");
+            }
+        }
+    }
+    // XXX: do we also need to call MatrixChat.setTheme here to do any random fixups (e.g. svg tint)
+
+
     if (window.localStorage && window.localStorage.getItem('mx_accepts_unsupported_browser')) {
         console.log('User has previously accepted risks in using an unsupported browser');
         validBrowser = true;