For a quick workaround you can use TamperMonkey to activate Dark Mode on websites by default:

1. Install Tampermonkey
   Go to: Tampermonkey for Opera
   Click Add to Opera

2. Create a New Script
   Click the Tampermonkey icon in your toolbar
   Choose Create a new script...

3. Paste the Script Below
   Replace the default template with this code:

// ==UserScript==
// @name         Force prefers-color-scheme: dark
// @namespace    http://tampermonkey.net/
// @version      1.0
// @description  Forces websites to use dark mode by overriding prefers-color-scheme
// @author       YourName
// @match        *://*/*
// @grant        none
// @run-at       document-start
// ==/UserScript==

(function() {
    'use strict';

    // Override matchMedia to force dark mode
    const originalMatchMedia = window.matchMedia;
    window.matchMedia = function(query) {
        if (query === '(prefers-color-scheme: dark)') {
            return {
                matches: true,
                media: query,
                onchange: null,
                addListener: function() {},
                removeListener: function() {},
                addEventListener: function() {},
                removeEventListener: function() {},
                dispatchEvent: function() { return false; }
            };
        }
        if (query === '(prefers-color-scheme: light)') {
            return {
                matches: false,
                media: query,
                onchange: null,
                addListener: function() {},
                removeListener: function() {},
                addEventListener: function() {},
                removeEventListener: function() {},
                dispatchEvent: function() { return false; }
            };
        }
        return originalMatchMedia(query);
    };

    // Optional: Set color-scheme via CSS as well
    const style = document.createElement('style');
    style.innerHTML = `
        html {
            color-scheme: dark !important;
        }
    `;
    document.documentElement.appendChild(style);
})();

4. Save the Script
   Click File > Save or use Ctrl + S

Now all websites that support dark mode should be dark by default.