Whenever I’m handcrafting CSS, content selection rules are usually the first declarations on my sheet. Might as well ensure user selected text is readable. Easiest win for accessibility.

::selection {
  color: #fff;
  background-color: #000;
}

::-webkit-selection {
  color: #fff;
  background-color: #000;
}

::-moz-selection {
  color: #fff;
  background-color: #000;
}