Just a guess, but have you tried using .post-avatar instead of .topic-avatar? This might be at least part of the problem (if not all of it).
On desktop, that former class became embedded in the latter during the course of this year and stopped the Christmas hats CSS working too (see related solution here: Christmas hats! - #19 by merefield)
Do always use your browser’s inspector to look at the HTML (right-click → Inspect). This can give you big hints as to what is going on!