Initial source: 100 Bytes of CSS to look great everywhere - https://dev.to/swyx/100-bytes-of-css-to-look-great-everywhere-19pd - or use https://watercss.kognise.dev/

style.css

html {
  max-width: 70ch;
  padding: 1rem;
  margin: auto;
  line-height: 1.75rem;
  font-size: 1rem;
  font-family: "Open Sans", Arial, sans-serif;
}

h1, h2, h3, h4, h5, h6 {
  margin-bottom: 1rem;
  margin-top: 3rem;
}

p, ul, ol {
  margin-bottom: 2rem;
  color: #1d1d1d;
}

pre {
  background: #f4f4f4;
  padding: 15px;
  border-radius: 10px;
  white-space: pre-wrap;
}