Here is a quick guide that shares two ways to apply and observe styling changes on any web page. 

Use a browser plugin

The "Live Editor for CSS" is a great Chrome extension that can be used with Chrome, Edge, Firefox, and Brave at the least. This extension makes it very easy to apply custom CSS and review the effects immediately. Works with less and sass, as well! 

Use the browser console

Sometimes an extension is not possible, or will not work for your situation. In those cases, we suggest pasting the CSS into your browser console. Of course, it expects JavaScript and not CSS, so you will need to wrap the CSS. For example: 

    const zpstyle = document.createElement('style');
    zpstyle.innerHTML = `
        /* Add your test CSS below */
        body {
            /* background-color: #f5f5f5;  Example: Change body background */
        }
    `;
    document.head.appendChild(zpstyle);