diff --git a/widget/nsNativeTheme.cpp b/widget/nsNativeTheme.cpp
index 3c53d84e965d..6044a0e1068b 100644
--- a/widget/nsNativeTheme.cpp
+++ b/widget/nsNativeTheme.cpp
@@ -30,6 +30,7 @@
 #include "mozilla/PresShell.h"
 #include "mozilla/StaticPrefs_layout.h"
 #include "mozilla/dom/DocumentInlines.h"
+#include "mozilla/RelativeLuminanceUtils.h"
 #include <algorithm>
 
 using namespace mozilla;
@@ -612,6 +613,28 @@ static nsIFrame* GetBodyFrame(nsIFrame* aCanvasFrame) {
   return body->GetPrimaryFrame();
 }
 
+bool nsNativeTheme::IsDarkColor(nscolor aColor) {
+  // Given https://www.w3.org/TR/WCAG20/#contrast-ratiodef, this is the
+  // threshold that tells us whether contrast is better against white or black.
+  //
+  // Contrast ratio against black is: (L + 0.05) / 0.05
+  // Contrast ratio against white is: 1.05 / (L + 0.05)
+  //
+  // So the intersection is:
+  //
+  //   (L + 0.05) / 0.05 = 1.05 / (L + 0.05)
+  //
+  // And the solution to that equation is:
+  //
+  //   sqrt(1.05 * 0.05) - 0.05
+  //
+  // So we consider a color dark if the contrast is below this threshold, and
+  // it's at least half-opaque.
+  constexpr float kThreshold = 0.179129;
+  return NS_GET_A(aColor) > 127 &&
+         RelativeLuminanceUtils::Compute(aColor) < kThreshold;
+}
+
 /* static */
 bool nsNativeTheme::IsDarkBackground(nsIFrame* aFrame) {
   // Try to find the scrolled frame. Note that for stuff like xul <tree> there
diff --git a/widget/nsNativeTheme.h b/widget/nsNativeTheme.h
index d565c839016a..d7f6bcef4fd9 100644
--- a/widget/nsNativeTheme.h
+++ b/widget/nsNativeTheme.h
@@ -185,13 +185,7 @@ class nsNativeTheme : public nsITimerCallback, public nsINamed {
   bool IsRangeHorizontal(nsIFrame* aFrame);
 
   static bool IsDarkBackground(nsIFrame* aFrame);
-  static bool IsDarkColor(nscolor aColor) {
-    // Consider a color dark if the sum of the r, g and b values is less than
-    // 384 in a semi-transparent document.  This heuristic matches what WebKit
-    // does, and we can improve it later if needed.
-    return NS_GET_A(aColor) > 127 &&
-           NS_GET_R(aColor) + NS_GET_G(aColor) + NS_GET_B(aColor) < 384;
-  }
+  static bool IsDarkColor(nscolor aColor);
 
   static bool IsWidgetScrollbarPart(mozilla::StyleAppearance aAppearance);