Custom stylesheet for Maintenance Mode plugin: https://wordpress.org/plugins/lj-maintenance-mode

maintenance.min.css

/**
 * maintenance.min.css
 */

/* Web Fonts */
@import url('https://fonts.googleapis.com/css2?family=Open+Sans:wght@400;700&display=swap');

/* Styles */
html {
  font-size: 16px;
}

body#error-page {
  padding: 4rem 2rem;
  margin: 0;
  max-width: none;
  background-color: #F1F1F1;
  border: none;
  box-shadow: none;
}

body#error-page .wp-die-message {
  padding: 1.5rem 2rem 1.75rem;
  margin: 0 auto;
  max-width: 700px;
  font-family: 'Open Sans', sans-serif;
  font-size: 1rem;
  background-color: #FFF;
  border: 1px solid #CCC;
  border-radius: 2px;
}

body#error-page h1 {
  padding: 0 0 0.5rem;
  margin: 0 0 1rem;
  font-size: 1.5rem;
  line-height: 1.2;
}

@media only screen and (min-width: 768px) {
  html {
    font-size: 20px;
  }

  body#error-page {
    padding-top: 6rem;
    padding-bottom: 6rem;
  }

  body#error-page .wp-die-message {
    font-size: 0.9rem;
  }
}