html, body {
  height: 100%;
  margin: 0;
}

#wrapper {
  display: flex;
  /* ensure wrapper fills the screen vertically */
  min-height: 100vh;
}

#sidebar-wrapper {
  /* fixed width you already have */
  width: 250px;
  /* make it span the entire viewport height */
  min-height: 100vh;
  /* optional: fix it in place */
  position: fixed;
  top: 0;
  left: 0;
  /* keep your bg/color rules */
}

#page-content-wrapper {
  /* push content over so it doesn’t sit underneath */
  margin-left: 250px;
  width: calc(100% - 250px);
}