Same here. The fix of deleting everything in the folder noted below and then restarting Teams worked for me for the past few days (since mid-week last week when I first noticed this issue and then found this thread with helpful suggestions), though I did have to restart Teams a few times before it would open and then stay open without the lines after deleting the contents of that folder.
%userprofile%\appdata\local\Packages\MSTeams_8wekyb3d8bbwe\LocalCache\Microsoft\MSTeams
The problem is, Teams kept showing me that it had an update available after I applied that fix, and I would have to go through the same steps each day after starting my laptop from scratch. Today, that fix stopped working. The lines are back, and I can't seem to get rid of them. I don't see a lot of info on this when I search about the issue, other than this thread.
Microsoft, this is a bad design choice. Please fix it. Those of us who choose dark mode do not want some weird hybrid of dark mode and high-contrast mode.