Adding a border to a button

I'm currently working on an options menu that has a couple of buttons: With 5 of them you can set the background color (black to white) and the others toggle other options like adding a coordinate system to the scene.

I already set the "normal", "highlighted" and "clicked" colors for all the buttons (they all use the standard "UISprite") but what I now want to do additionally is:

1. Give the button for the black background color (it's a narrow button UI object, no text) a white border, so you can actually see it, since the menus in my game have a simple, black background.

2. If an option gets selected, I want to give the according button a red border that stays/gets re-enabled after re-opening the options menu - until the option is disabled again.

Unfortunately the "Outline" component only seems to work with text. Is there another way to add a cheap border (in the UI for 1. and through a script for 2.)?

I'm using version 2017.3.1f1.

 

Anyone? :/