[Theme] Dark high contrast color theme

The existing dark themes all have fairly low contrast, which can make
text hard to read. This diff adds a variant of Darker-blue that
increases contrast between text and background.

src/Gui/Stylesheets/CMakeLists.txt

@@ -9,6 +9,7 @@ SET(Stylesheets_Files
     "Light-blue.qss"
     "Light-green.qss"
     "Light-orange.qss"
+    "Dark-contrast.qss"
 )
 
 # Find all the image files