@fluentui/react/dist/sass/_highContrast.scss

@mixin high-contrast {
  @media screen and (-ms-high-contrast: active) {
    @content;
  }
}

@mixin high-contrast-black-on-white {
  @media screen and (-ms-high-contrast: black-on-white) {
   @content;
  }
}

@mixin high-contrast-white-on-black {
  @media screen and (-ms-high-contrast: white-on-black) {
   @content;
  }
}