Minimalist Wikipedia theme (hide header and sidebars + bigger fonts + limit max width)

minimal_wiki.css

#content {
  margin-left: 0;
}

#mw-panel {
  display: none;
}

#bodyContent {
  font-size: 18px;
  max-width: 1024px;
  margin-left: auto;
  margin-right: auto;
}

#firstHeading {
  max-width: 1024px;
  margin-left: auto;
  margin-right: auto;
}

.header-toggle-container {
  z-index: 999;
  position: absolute;
  top: 0;
  left: 0;
}

.header-toggle-btn {
  font-size: 1rem;
  background-color: inherit;
  border: 0;
  cursor: pointer;
  outline: none;
}

minimal_wiki.js

const head = document.getElementById("mw-head");
const pageBase = document.getElementById("mw-page-base");

function setHeader(show) {
  const config = show ? "block" : "none";
  head.style.display = config;
  pageBase.style.display = config;
}

var isShowing = false;

setHeader(isShowing);

const container = document.createElement('div');
const toggleBtn = document.createElement('button');
const label = document.createTextNode('-');
container.className = "header-toggle-container";
toggleBtn.className = "header-toggle-btn";
container.appendChild(toggleBtn);
toggleBtn.appendChild(label);
toggleBtn.onclick = function () {
  setHeader(isShowing = !isShowing);
};

document.body.insertBefore(container, pageBase);