User blog comment:Rupert Giles/Prototype for a new article layout/@comment-6532857-20150218025057/@comment-24416108-20150220232302

Most infoboxes are just a kind of tables. You could try some table CSS, which is one of the things still allowed on the wiki-wide CSS page.

Or I could have misunderstood your question, since " a fix to infobox borders?" is rather vague.