diff --git a/widget/gtk/nsLookAndFeel.cpp b/widget/gtk/nsLookAndFeel.cpp
index 562eaa97b7..d2b82e4957 100644
--- a/widget/gtk/nsLookAndFeel.cpp
+++ b/widget/gtk/nsLookAndFeel.cpp
@@ -75,6 +75,7 @@ ApplyColorOver(const GdkRGBA& aSource, GdkRGBA* aDest) {
         aDest->red = sourceCoef * aSource.red + destCoef * aDest->red;
         aDest->green = sourceCoef * aSource.green + destCoef * aDest->green;
         aDest->blue = sourceCoef * aSource.blue + destCoef * aDest->blue;
+        aDest->alpha = resultAlpha;
     }
 }