StyleSwitcher is a easy way to change the css in greasemonkey.

StyleSwitcher.js

// ==UserScript==
// @name               StyleSwitcher
// @namespace          https://github.com/cologler/
// @version            0.2.2
// @description        try to take over the world!
// @author             cologler
// @grant              GM_addStyle
// @grant              GM_getValue
// @grant              GM_setValue
// @grant              GM_info
// @grant              GM_unregisterMenuCommand
// @grant              GM_registerMenuCommand
// ==/UserScript==

// just let type script work.
(function() { function require(){}; require("greasemonkey"); })();

class _Style {
    constructor(name, css) {
        this.style = GM_addStyle(css);
        Object.defineProperty(this, 'name', { get: () => name });

        this.disable();
    }

    enable() {
        this.style.disabled = false;
    }

    disable() {
        this.style.disabled = true;
    }
}

class StyleSwitcher {
    constructor(storeKey = 'cssstyle') {
        this._csses = [];
        this._enabled = null;
        this._cmdId = null;
        const name = GM_info.script.name_i18n[navigator.language.replace('-', '_')] || GM_info.script.name;
        Object.defineProperty(this, 'name', { get: () => name })
        Object.defineProperty(this, 'storeKey', { get: () => storeKey });
        let style = null;
        Object.defineProperty(this, 'currentStyle', {
            get: () => style,
            set: value => {
                value.enable();
                if (style !== null) {
                    style.disable();
                }
                style = value;
            }
        })
    }

    addStyle(name, css) {
        this._csses.push(new _Style(name, css));
    }

    load() {
        if (this._csses.length == 0) {
            return;
        }
        let i = this.getIndex();
        this.apply(i);
    }

    getIndex() {
        return GM_getValue(this.storeKey, 0);
    }

    setIndex(value) {
        GM_setValue(this.storeKey, value);
    }

    apply(index) {
        if (this._cmdId !== null) {
            GM_unregisterMenuCommand(this._cmdId);
            this._cmdId = null;
        }

        this.currentStyle = this._csses[index % this._csses.length];
        let nextStyle = this._csses[(index + 1) % this._csses.length];

        if (this.currentStyle !== nextStyle) {
            let self = this;
            let menu = `${this.name} switch to ${nextStyle.name}`;
            this._cmdId = GM_registerMenuCommand(menu, () => {
                if (self._csses.length == 0) {
                    return;
                }
                let i = GM_getValue(self.storeKey, 0);
                i++;
                self.setIndex(i % self._csses.length);
                self.apply(i);
            });
        }
    }
}

~README.md

switch css style.

let sw = new StyleSwitcher();
sw.addStyle('hide all',
`
* { display: none }
`);
sw.load();

now you can switch css on menu command.