The override list is at the URL opera://settings/content/forceDarkMode.

In regular Opera (as long as you have the flag at the URL opera://flags/#enable-force-dark-from-settings enabled), if you have the "Force dark theme on pages" option enabled and the theme set to "dark" at the URL opera://settings/appearance, you'll see the right-click option to toggle dark mode for the page, which will modify the override list.

In Opera GX, as long as the theme mode is set to dark, you'll see the right-click option for pages regardless of whether the "Force dark pages" option is enabled at the URL opera://settings/opera-gx. Just think of "Force dark pages" as the default, global option for pages where the right-click option for a page allows you to override that global.

Either way, all the options are showing fine for me.