UserScript to invert colors on-click which only is reminded for current tab. Should be added to UserScript manager (tested and working in Firemonkey+Firefox). Present in all sites, except in the ones matched by @ exclude

InvertCss.js

// ==UserScript==
// @name        InvertCSS
// @exclude						 https://*.github.com/*
// @match    *://*/*
// ==/UserScript==

function addGlobalStyle(css) {
    let head, style;
    head = document.getElementsByTagName('head')[0];
    if (!head) { return; }
    style = document.createElement('style');
    style.type = 'text/css';
    style.innerHTML = css;
    head.appendChild(style);
}


/* Checks if the button was clicked in the current tab */
let cat = sessionStorage.getItem("myCat");

if (cat === "Tom") {
  addGlobalStyle(`@media (prefers-color-scheme: dark) {
  body {
    filter: invert(100%);
    background-color: rgb(25, 25, 25) !important;
  }
  img,video,
  iframe /* for recaptcha or web embed */ {
    filter: invert(100%) !important;
  }
}
@media (prefers-color-scheme: light) {
  body {
    background-color: #fff !important;
    color: black !important;
  }}

`);
}


/* Button creation and position */
var button = document.createElement("Button");
button.innerHTML = "Force Colorscheme";
button.style = "top:0;right:0;position:absolute;z-index:99999;padding:6px;";
document.body.appendChild(button);



// add click event listener
button.addEventListener('click', () => {
addGlobalStyle(`@media (prefers-color-scheme: dark) {
  body {
    filter: invert(100%);
    background-color: rgb(25, 25, 25) !important;
  }
  img,
  iframe /* for recaptcha or web embed */ {
    filter: invert(100%) !important;
  }
}
@media (prefers-color-scheme: light) {
  body {
    background-color: #fff !important;
    color: black !important;
  }}
`);

/* We save this value to remember the click in the current tab */
sessionStorage.setItem("myCat", "Tom");
});

Example( wikipedia).

---

PS: This script follows the color of the browser: if the browser is using a light color, and you click the button inside a dark site, it will invert the site's color to lighter ones. If you browser is using a dark theme, and you click the button inside a white/light site it will make the colors to become darker(Like in the picture).

This will not make a dark site to become lighter if you browser is in dark mode, and it will not turn dark a white site if you are using a light theme.