I meant to invert the fore color, not the back color. cleanup for my previous patch. b=338374

gfx/src/mac/nsRenderingContextMac.cpp

@@ -887,9 +887,10 @@ NS_IMETHODIMP nsRenderingContextMac::DrawLine(nscoord aX0, nscoord aY0, nscoord
       ::PenPat(&dashedPattern);
     else
       ::PenPat(&dottedPattern);
-    RGBColor invertedColor = savedBGColor;
-    ::InvertColor(&invertedColor);
-    ::RGBBackColor(&invertedColor);
+    RGBColor invertedForeColor;
+    ::GetForeColor(&invertedForeColor);
+    ::InvertColor(&invertedForeColor);
+    ::RGBBackColor(&invertedForeColor);
   }
 
 	mGS->mTMatrix.TransformCoord(&aX0,&aY0);
@@ -936,9 +937,10 @@ NS_IMETHODIMP nsRenderingContextMac::DrawPolyline(const nsPoint aPoints[], PRInt
       ::PenPat(&dashedPattern);
     else
       ::PenPat(&dottedPattern);
-    RGBColor invertedColor = savedBGColor;
-    ::InvertColor(&invertedColor);
-    ::RGBBackColor(&invertedColor);
+    RGBColor invertedForeColor;
+    ::GetForeColor(&invertedForeColor);
+    ::InvertColor(&invertedForeColor);
+    ::RGBBackColor(&invertedForeColor);
   }
 
   PRInt32 x,y;