You use extended layout (this is folder "1" in layouts), so you need to change forum.php in this folder layouts/1/forum.php.

Now is working?

P.S. Topic title length depends only from code, which I wrote above. 

Is there a way to setup a wider topic string for desktop users (say 80 chars) and a narrow layout for mobile users (say 52 chars)? Sort of what you can do in CSS:

/* mobile devices */
@media only screen and (max-width: 1024px){ [topic-string=52chars] }

/* desktop devices */
@media only screen and (min-width: 1024px){ [topic-string=80chars] }

No way yet. It's being hard cropped in core functions. I'll add this in our to-do list.