You'll need to find the ID of the tag identified as "Forum Games" then add the same CSS code I provided earlier. If you let me know the URL for your site, I can provide the actual code you need. Note, that if you move the tags around, the bottom border will follow it as it's associated to the tag ID.
EDIT - actually, just reading this again, for that to work, you'd need an event to bind it to and would probably have to use JS (JavaScript). The problem with this is that any JS will execute on page load, but because Flarum is an SPA (Single Page Application), you'd need an extension (or perhaps leverage extend.php) to perform such a task (I think - perhaps the devs can clarify)
