How can I inject CSS into a web page using JavaScript?

I've often found myself in need of dynamically modifying CSS on a web page. While it's very easy to alter the styles of individual elements, using Element.style, it's not as easy to inject a whole new set of styles into the page. Being able to do so comes particularly handy when working with CSS variables.
Thinking a little outside the box, we might consider how we usually write CSS - in a <style> tag. This is exactly what we'll be doing in order to inject some new CSS into the page.
Using Document.createElement(), we can create a new style element and set its type to text/css. We can then set its innerText to the CSS string we want to inject. Finally, we can use Document.head and Node.appendChild() to append the new element to the document head.
Doing so will make the browser parse the CSS and apply it to the page. The newly created style element will be returned, in case you need to keep a reference to it.

const injectCSS = css => {
  let el = document.createElement('style');
  el.type = 'text/css';
  el.innerText = css;
  document.head.appendChild(el);
  return el;
};

injectCSS('body { background-color: #000 }');
// '<style type="text/css">body { background-color: #000 }</style>'

Using Document.createElement(), we can create a new style element and set its type to text/css. We can then set its innerText to the CSS string we want to inject. Finally, we can use Document.head and Node.appendChild() to append the new element to the document head.
Doing so will make the browser parse the CSS and apply it to the page. The newly created style element will be returned, in case you need to keep a reference to it.

From 30-seconds-of-code: inject-css