sandbox: Select white-on-black

Use white on black for the expo menu as it is easier on the eyes.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 52e9ddb..7026c73 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -131,6 +131,7 @@
 			font-size = <30>;
 			menu-inset = <3>;
 			menuitem-gap-y = <1>;
+			white-on-black;
 		};
 
 		cedit-theme {