[CRX] Use dark theme by default in Chrome extension

Default to the dark theme for consistency with the previous version of the
published extension (was 12ff252751).

extensions/chromium/preferences_schema.json

@@ -10,7 +10,7 @@
         1,
         2
       ],
-      "default": 0
+      "default": 2
     },
     "showPreviousViewOnLoad": {
       "description": "DEPRECATED. Set viewOnLoad to 1 to disable showing the last page/position on load.",

web/app_options.js

@@ -177,7 +177,7 @@ const defaultOptions = {
   },
   viewerCssTheme: {
     /** @type {number} */
-    value: 0,
+    value: typeof PDFJSDev !== "undefined" && PDFJSDev.test("CHROME") ? 2 : 0,
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
   viewOnLoad: {