User:TheMcr/monobook.js

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// Create a new <style> element
var customStyle = document.createElement("style");
customStyle.innerHTML = `
   body {
  font: x-small sans-serif;
  background: #990000 url(/wiki/skins/monobook/headbg.jpg) 0 0 no-repeat;
  color: rgb(255, 255, 255);
  margin: 0;
  padding: 0;
}
`;

// Append the <style> element to the document head
document.head.appendChild(customStyle);
console.log("Custom CSS applied via JavaScript!");