You probably have the “Draw As” parameter in your hover set to “Box”. Instead create a png image that is just a box with a hollow center and use that. Set the “Draw As” value to “Image” in this case.
You could also set the value to “Border”, in fact this is supposed to do exactly what you are after, but it is giving me something really funky in 4.8 which I expect to be a bug. When I set “Draw as” to “Border” in 4.8 I get this when hovering over the button:
The keyboard/controller selected tile still looks the same. I’m trying to get the hovered effect that hovering the mouse cursor over the button gives the button, where it looks bordered. I have an image on top of the button with the material image so having “draw as” box works fine for mouse cursor, i’m just trying to get the same thing for keyboard/controller selected.