Unfortunately the source code is unavailable as well as the APIs, otherwise many of us would have already re-skinned it.
But we don’t use the chat as much as you would in slack, because it’s totally broken. We mostly just take lots of videos calls and share notes with an internal pastebin/gist tool.
saw the discussion in another thread for softwares used in medical, not easy to use, one reason someone mentioned is that the purchaser does not use it much, the real users(doctors/nurses)' voice are probably not heard enough.
wondered about some ux inconveniences in teams, if MS use it internally, should have improved it... :-)
probably not fair to compare Teams and slack, as you said, MS has more things to consider for Teams than slack... while seems even some minor things, it is kind of annoying, can be easily fixed, but stay like that for long time... thinking if the Teams product team folks use the product and care, they should have fixed/improved...
seems this is common for MS products, powerful, many functions, but do not care enough for UX..., and one of things makes it different to apple :-)