Use gray and white as the default dialog theme (#637)

I think this has a few benefits:

1. Most terminal themes generally don't change or pick similar colors
   for their black and white colors, so it's more likely to be consistent
    across terminals/themes.
2. They don't have the potential color-blind issues that other color options
   may have.
3. We won't need additional changes for no color mode.

lib/reline/face.rb

@@ -186,9 +186,9 @@ def self.load_initial_configs
       conf.define :scrollbar, style: :reset
     end
     config(:completion_dialog) do |conf|
-      conf.define :default, foreground: :white, background: :cyan
-      conf.define :enhanced, foreground: :white, background: :magenta
-      conf.define :scrollbar, foreground: :white, background: :cyan
+      conf.define :default, foreground: :bright_white, background: :gray
+      conf.define :enhanced, foreground: :black, background: :white
+      conf.define :scrollbar, foreground: :white, background: :gray
     end
   end