[Ide][Mac] Default button label must be white with dark theme

Author: sevoku

main/src/core/MonoDevelop.Ide/gtkrc.mac-dark

@@ -172,6 +172,12 @@ style "button" = "default" {
 style "dialog-button" = "button" {
     GtkButton::inner-border = { 7, 7, 3, 4 }
 
+    text[NORMAL] = @selected_fg_color
+    text[PRELIGHT] = @selected_fg_color
+    text[SELECTED] = @selected_fg_color
+    text[INSENSITIVE] = @dim_color
+    text[ACTIVE] = @selected_fg_color
+
     engine "xamarin" {
         default_button_color = "#2d65d3"
     }