Add CSS rules quickly from the browser JS console. Idea taken from http://davidwalsh.name/add-rules-stylesheets

console-paste-styles.js

(() => {
  var style = document.createElement("style");
  style.appendChild(document.createTextNode(""));
  document.head.appendChild(style);
  var rules = prompt('Paste CSS rules here');
  var importStatements = (rules.split(';') || []).filter(s => s.startsWith('@import')).map(s => s + ';');
  importStatements.forEach((importStatement, index) => style.sheet.insertRule(importStatement, index));
  style.sheet.insertRule(`@media all{${rules}`, importStatements.length);
}())

Bookmarklet:

javascript:(()=>{var e=document.createElement("style");e.appendChild(document.createTextNode(""));document.head.appendChild(e);var t=prompt('Paste CSS rules here'),r=(t.split(';')||[]).filter(e=>e.startsWith('@import')).map(e=>e+';');r.forEach((t,r)=>e.sheet.insertRule(t,r));e.sheet.insertRule(`@media all{${t}`,r.length)})()