Version 78.0 changed the default app for all of my .MKV files and their icons to MKVToolNix
Upon first installation of MKVToolNix 78.0, any .MKV files I opened were automatically added to a new MKVToolNix window. I chose File->Get Info for the MKV file, and indeed, the default "Open with:" application had changed itself to MKVToolNix.
I manually changed this back to my preferred app (VLC) and then selected "Change All." This did fix the default app, but now all of my MKV files have the MKVToolNix icon on them. I restarted the machine. I used OnyX to rebuild the LaunchServices database. I restarted again. All MKV files still have the MKVToolNix icon.
Running Mac OS Ventura 13.4.1 on a Mac Studio with M1 Max