"Online play is not rated" checkbox has no outline when focused

The bug

When opening the multiplayer menu for the first time (or setting skipMultiplayerWarning:false in the options.txt file) a warning screen is shown.
The checkbox on that warning screen has no outline (like buttons do) when focused, decreasing accessibility.

How to reproduce

1. Click on "Multiplayer"
2. On the warning screen navigate using the TAB key
   The checkbox can have focus (can be toggled using SPACE), but it has no outline