Fix mark styles in Windows High Contrast Mode

In some browsers (IE11, Chromium-based browsers) the marked text in
search results are unreadable with forced colour modes when the
background is black because the text colour for `<mark>` elements is
set to black.

This commit fixes this by re-asserting that the marked text should
be the same colour as the surrounding text.

We also have to set `background-color: Canvas`, because in Firefox
`background-color: transparent` is also ignored (unlike in IE11,
Chromiums).

lib/assets/stylesheets/modules/_search.scss

@@ -162,4 +162,20 @@ html.has-search-results-open {
   background-color: transparent;
   color: inherit;
   font-weight: bold;
+
+  // In forced color mode some browsers will keep the background transparent
+  // but set the text colour black, making the highlighted text unreadable.
+  // The following blocks fix this by setting the text colour to be the same as
+  // other text in a way that forced color mode will respect.
+
+  @media screen and (-ms-high-contrast: active) {
+    // IE does not support `CanvasText`,
+    // and `currentColor` does not work with Blink
+    color: currentColor;
+  }
+
+  @media screen and (forced-colors: active) {
+    background-color: Canvas;  // needed for Firefox
+    color: CanvasText;
+  }
 }