[CRX] Add option to customize Theme in options UI

The `viewerCssTheme` option was not rendered because its entry in
`preferences_schema.json` did not have a `title`.

The order of keys in `preferences_schema.json` determines the order of the
rendered preferences in the options UI. Since `viewerCssTheme` affects the UI
very significantly, I have moved the option to the top.

extensions/chromium/options/options.html

@@ -43,6 +43,19 @@ body {
 </div>
 </template>
 
+<template id="viewerCssTheme-template">
+<div class="settings-row">
+  <label>
+    <span></span>
+    <select>
+      <option value="0">Use system theme</option>
+      <option value="1">Light theme</option>
+      <option value="2">Dark theme</option>
+    </select>
+  </label>
+</div>
+</template>
+
 <template id="viewOnLoad-template">
 <div class="settings-row">
   <label>

extensions/chromium/preferences_schema.json

@@ -1,6 +1,17 @@
 {
   "type": "object",
   "properties": {
+    "viewerCssTheme": {
+      "title": "Theme",
+      "description": "The theme to use.\n0 = Use system theme.\n1 = Light theme.\n2 = Dark theme.",
+      "type": "integer",
+      "enum": [
+        0,
+        1,
+        2
+      ],
+      "default": 0
+    },
     "showPreviousViewOnLoad": {
       "description": "DEPRECATED. Set viewOnLoad to 1 to disable showing the last page/position on load.",
       "type": "boolean",
@@ -188,15 +199,6 @@
         2
       ],
       "default": -1
-    },
-    "viewerCssTheme": {
-      "type": "integer",
-      "enum": [
-        0,
-        1,
-        2
-      ],
-      "default": 0
     }
   }
 }