:root {
  --radius-of-border: 7px;
  --header-box-width: 70vw;
  --url-hover-purple: #dea3d6;
  --page-background: #353839;
  --cyan: #5bbcc8;
  --blue-soft: #5cc1cd;
  --green-cyan: #85c495;
  --green-pale: #50FA7B;
  --grey-dark: #adadad;
  --grey-light: #c2c3c4;
  --purple-light: #caa9fa;
  --red-clay: #b36b65;
}
body {
  background: var(--page-background);
  color: var(--grey-light);
  white-space: normal;
  font-size: 18px;
  margin-top: .5vw;
  margin-left: 5%;
  margin-right: 5%;
}
#centralTable {
  width: 70vw;
}
a:link {
  color: var(--blue-soft);
}
a:hover {
  color: var(--url-hover-purple);
}
h1 {
  color: var(--cyan);
}
h2 {
  font-weight: bold;
  color: var(--purple-light);
}
h3 {
  font-weight: bold;
  color: var(--red-clay);
  font-size: 40px;
}
pre {
  color: var(--grey-light);
  font-size: 20px;
  white-space: pre-wrap;
}
/* detail description modifications found from
 * https://stackoverflow.com/questions/55031395/how-to-change-summary-text-if-details-has-open-attr
 */
details.description[open] summary::after {
  content: attr(data-open);
}
details.description:not([open]) summary::after {
  content: attr(data-close);
}
details[open] summary {
  color: var(--green-pale);
}
summary {
  width: max-content; /* ensure clickable area only as long as the text */
  text-decoration: underline;
  cursor: pointer;
  color: var(--grey-dark);
}
ul li select {
  color: var(--green-cyan);
  background: black;
  border: none;
  font-size: 30px;
  text-decoration:  underline;
}
#mainPageHeaderBox {
  background: black;
  width: var(--header-box-width);
  border-radius: var(--radius-of-border);
  padding: .05%;
}
#mainPageHeaderBox ul {
  padding-left: 0px;
}
#mainPageHeaderBox ul li {
  display: inline-block;
  font-size: 30px;
}
#mainPageHeaderBox a:link {
  color: var(--green-cyan);
}
#mainPageHeaderBox a:hover {
  color: var(--url-hover-purple);
}
#aboutBubble {
  border-radius: var(--radius-of-border);
  color: lime;
  background-color: black;
  padding: .05%;
  width: var(--header-box-width);
}
/*Floating Back-To-Top Button https://stackoverflow.com/questions/32102747/how-to-make-a-back-to-top-button-using-css-and-html-only */
#returnToTop {
  position: fixed;
  bottom: 10px;
  left: 95%;
  max-width: 50px;
  width: 100%;
  font-size: 12px;
  background-color: transparent;
  border-color: transparent;
  color: lime;
}
<meta name="viewport" content="width=device-width, initial-scale=1.0">