Attempt to fix focus ring being cut off in table of contents

[mdo]

Potential alternative to #37940.

/cc @patrickhlauke

[patrickhlauke]

This also works, sure. I wasn't sure whether not having the little left-hand border on the active item on medium size was intentional or not, so my approach kept that behaviour (hence the need for the media queries). This is obviously much more straightforward, and keeps the left-hand border at medium, large, and extra large sizes