121870: adjust threshold for <Color::IsDark()> to improve interoperability with…

121870: adjust threshold for <Color::IsDark()> to improve interoperability with Microsoft Office documents

Patch by: Pedro <pfg@apache.org>
Review by: Oliver <orw @apache.org>

tools/source/generic/color.cxx

@@ -143,7 +143,7 @@ void Color::Invert()
 
 sal_Bool Color::IsDark() const
 {
-    return GetLuminance() <= 38;
+    return GetLuminance() <= 55;
 }
 
 // -----------------------------------------------------------------------