I think the UI is way too small and most of the space is not utilized. I also don't like how the left side menu pushes everything over to the right for larger screens as well, there's no need for that. For smaller screens, It should be fine, but not for larger screens.
I mean, this is what it looks like:
https://i.imgur.com/7dPR2tJ.jpg
If you don't have the time, I would be willing to style this per your request for free.
And I fidgeted around and this is what it could look like (On the same 2K resolution):
https://i.imgur.com/8t9HoKt.jpg
P.S. I know that the background image cover is not adjusted properly, but that shouldn't be a problem.
P.P.S. I would also like to see another filter: "Highlight user message" where you can choose to highlight a user's message. I know It's not necessary, but I think It's a nice feature to kind of stand out users from the general chat.