Gist for this blogpost: https://asontu.github.io/2024/05/16/a-poor-mans-custom-stylesheet-bookmarklet.html

Poor man's custom css bookmarklet.js

javascript:(function(){
    var styleTag = document.querySelector('#niftyStyle');
    if (!styleTag) {
        styleTag = document.createElement('style');
        styleTag.id = 'niftyStyle';
        styleTag.type = 'text/css';
        styleTag.contentEditable = 'plaintext-only';
        styleTag.style.position = 'fixed';
        styleTag.style.top = '0';
        styleTag.style.minHeight = '1em';
        styleTag.style.minWidth= '4em';
        styleTag.style.zIndex = '99999';
        styleTag.style.padding = '3px';
        styleTag.style.whiteSpace = 'pre';
        styleTag.style.background = '#000';
        styleTag.style.color = '#fff';
        styleTag.style.fontFamily = 'consolas, monospace';
        document.body.appendChild(styleTag);
    }
    if (!styleTag.innerText.trim()) {
        var sheet = localStorage.getItem('niftyStyle');
        if (!sheet) {
            sheet = 'html {\n    filter: invert(1) hue-rotate(180deg);\n}';
            styleTag.style.display = 'block';
        }
        styleTag.appendChild(document.createTextNode(sheet));
        return;
    }
    if (styleTag.style.display == 'block') {
        localStorage.setItem('niftyStyle', styleTag.innerText.trim());
        styleTag.style.display = 'none';
    } else {
        styleTag.style.display = 'block';
    }
})();