Updated alpha value for disabled icons in default theme.

doc/classes/Button.xml

@@ -115,7 +115,7 @@
 		<theme_item name="font_pressed_color" data_type="color" type="Color" default="Color(1, 1, 1, 1)">
 			Text [Color] used when the [Button] is being pressed.
 		</theme_item>
-		<theme_item name="icon_disabled_color" data_type="color" type="Color" default="Color(1, 1, 1, 1)">
+		<theme_item name="icon_disabled_color" data_type="color" type="Color" default="Color(1, 1, 1, 0.4)">
 			Icon modulate [Color] used when the [Button] is disabled.
 		</theme_item>
 		<theme_item name="icon_focus_color" data_type="color" type="Color" default="Color(1, 1, 1, 1)">

scene/resources/default_theme/default_theme.cpp

@@ -175,7 +175,7 @@ void fill_default_theme(Ref<Theme> &theme, const Ref<Font> &default_font, Ref<Te
 	theme->set_color("icon_hover_color", "Button", Color(1, 1, 1, 1));
 	theme->set_color("icon_hover_pressed_color", "Button", Color(1, 1, 1, 1));
 	theme->set_color("icon_focus_color", "Button", Color(1, 1, 1, 1));
-	theme->set_color("icon_disabled_color", "Button", Color(1, 1, 1, 1));
+	theme->set_color("icon_disabled_color", "Button", Color(1, 1, 1, 0.4));
 
 	theme->set_constant("hseparation", "Button", 2 * scale);