Add a new preference, viewerCssTheme, to allow forcing the use of the light/dark viewer CSS themes (issue 12290)

While this does work pretty well in my quick testing, it's *very much* a hack since as far as I can tell there's no support in the CSS specification for using e.g. a CSS variable to override a `@media (prefers-color-scheme: dark) {...}` block.

The solution implemented here is thus to *edit* the viewer CSS, by either removing the entire `@media ...` block in light-mode or by ensuring that its rules become *unconditionally* applied in dark-mode.
To simplify the overall implementation, since all of this does seem like somewhat of an edge-case, the `viewerCssTheme` preference will *only* be read during viewer initialization. (Similar to many other existing preferences, a reload is thus required when changing it.)

web/app_options.js

@@ -154,6 +154,11 @@ const defaultOptions = {
     value: false,
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
+  viewerCssTheme: {
+    /** @type {number} */
+    value: 0,
+    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
+  },
   viewOnLoad: {
     /** @type {boolean} */
     value: 0,