Forked from Eric Bailey's pen for this article: https://css-tricks.com/accessible-svgs-high-contrast-mode/
Changed to use CSS system color keywords to access the exact button colors used in the high contrast theme.
However, all the default themes in Windows use the same color for button backgrounds as they do for the page background. This makes the button not very obvious, as Eric noted in his article, if you just let default colors apply. I've adapted for that by adding a border on all sides of the button, so it looks more like other buttons in the application.
This is what it looks like in action: