First place to look is in the Accessibility section of your Settings. In the Visibility Enhancements area you'll see a 'show button shapes' option.
If that one doesn't do it, then UI Element borders are activated in the Developer Options (which need, themselves, be activated by doing certain steps). This would show borders around mostly everything on the UI, though...
If you didn't do it there nor activated Developer Options, it might be if your using an older theme not made for Oreo versions.