I think there are only very very rare cases where hacking core of MediaWiki is justificable/acceptable. Perhaps there is a hook or something that allows to do the task without amending core files.
Topic on Extension talk:BootStrapSkinContactForm
Sorry, I'm lost, where have I modified the core? To my knowledge I haven't touched any other files? Unless I've missed something?
Oops, I was having a senior moment here (since I am in my 40ies, too). I figured "SpecialContact.php" was a core file for some reasons. No worries.