Tweak the disabled files text color in FileDialog for readability

Contrast rate is still fairly low, but the text needs to be easy enough
to distinguish from non-disabled items.

doc/classes/FileDialog.xml

@@ -130,7 +130,7 @@
 		<theme_item name="file_icon_modulate" data_type="color" type="Color" default="Color(1, 1, 1, 1)">
 			The color modulation applied to the file icon.
 		</theme_item>
-		<theme_item name="files_disabled" data_type="color" type="Color" default="Color(0, 0, 0, 0.7)">
+		<theme_item name="files_disabled" data_type="color" type="Color" default="Color(1, 1, 1, 0.25)">
 			The color tint for disabled files (when the [FileDialog] is used in open folder mode).
 		</theme_item>
 		<theme_item name="folder_icon_modulate" data_type="color" type="Color" default="Color(1, 1, 1, 1)">

scene/resources/default_theme/default_theme.cpp

@@ -605,7 +605,7 @@ void fill_default_theme(Ref<Theme> &theme, const Ref<Font> &default_font, const
 	theme->set_icon("file", "FileDialog", icons["file"]);
 	theme->set_color("folder_icon_modulate", "FileDialog", Color(1, 1, 1));
 	theme->set_color("file_icon_modulate", "FileDialog", Color(1, 1, 1));
-	theme->set_color("files_disabled", "FileDialog", Color(0, 0, 0, 0.7));
+	theme->set_color("files_disabled", "FileDialog", Color(1, 1, 1, 0.25));
 
 	// Popup