Use main theme color for text selection

[daun]

The new default admin theme doesn't override UiKit's selection color, resulting in all text selections using the default UiKit blue.

One solution would be to use the main color as selection color. The other would be to reset this and leave it to the browser to apply a system color.

::selection {
  background: var(--main-color);
  color: #fff;
  text-shadow: none;
}

[adrianbj]

Weirdly I am getting an unknown green - see second part of my initial post: #2089