Update README.

1 files changed, 36 insertions, 2 deletions

diff --git a/macosx/README b/macosx/README
index c79c372..cca90c5 100644
--- a/macosx/README
+++ b/macosx/README
@@ -285,8 +285,42 @@ ensure that we maintain consistency, changing the tabbingIdentifier of a window
 which is already displayed as a tab will also cause it to become a separate
 window.
 
-
-4. Building Tcl/Tk on macOS
+4. Ttk, Dark Mode and semantic colors
+---------------------------------------
+
+With the release of OSX 10.14 (Mojave), Apple introduced the DarkAqua
+appearance.  Part of the implementation of the Dark Mode was to make some of
+the named NSColors have dynamic values.  Apple calls these "semantic colors"
+because the name does not specify a specific color, but rather refers to the
+context in which the color should be used.  Tk now provides the following
+semantic colors as system colors: systemTextColor, systemTextBackgroundColor,
+systemSelectedTextColor, systemSelectedTextBackgroundColor,
+systemControlTextColor, systemDisabledControlTextColor, and systemLabelColor.
+All of these except systemLabelColor, which was introduced in 10.10, were
+present in OSX 10.0. The change in 10.14 was that the RGB color value of these
+colors became dynamic, meaning that the color value can change when the
+application appearance changes.  In particular, when a user selects Dark Mode
+in the system preferences these colors change appearance.  For example
+systemTextColor is dark in Aqua and light in DarkAqua.
+
+The default background and foreground colors of most of the Tk widgets
+have been set to semantic colors, which means that the widgets will change
+appearance, and remain usable, when Dark Mode is selected in the system
+preferences.  However, to get a close match to the native Dark Mode style it
+is recommended to use Ttk widgets when possible.
+
+Apple's tab view and GroupBox objects delimit their content by
+displaying it within a rounded rectangle with a background color that
+contrasts with the background of the containing object.  This means
+that the background color of a Ttk widget depends on how deeply it is
+nested inside of other widgets that use contrasting backgrounds.  To
+support this, there are 8 contrasting system colors named
+systemTtkBackground, systemTtkBackground1, ... , systemTtkBackground7.
+The systemTtkBackground color is the standard background for a dialog
+window and the others match the contrasting background colors for
+backgrounds which are nested to the corresponding depth.
+
+5. Building Tcl/Tk on macOS
 ------------------------------
 
 - At least macOS 10.3 is required to build Tcl and TkX11, and macOS 10.6