message: Fix ActionButton highlighting on macOS < 10.14
  The button had opaque black background instead of slight tint on mouse
  hover.
diff: NSColor *bg = self.window.backgroundColor;
  if (self.mouseHover)
  {
  - NSColor *highlight;
  if (@available(macOS 10.14, *)) {
  - highlight = [bg colorWithSystemEffect:NSColorSystemEffectRollover];
  - } else {
  - highlight = [NSColor colorWithWhite:0.72 alpha:1.0];
  - }
  + NSColor *highlight = [bg colorWithSystemEffect:NSColorSystemEffectRollover];
  // use only lighter version of the highlight by blending with the background
  bg = [bg blendedColorWithFraction:0.2 ofColor:highlight];
  + } else {
  + bg = [NSColor colorWithWhite:0.9 alpha:1.0];
  + }
  }
  [StyleKit drawActionButtonWithFrame:self.bounds
  buttonColor:bg
  