I've found a solution that seems to work while still using dark mode. If you go to to flags using opera://flags then click on the drop down box next to "Auto dark mode for Web-Contents" and select "Enabled with selective inversion of non-image elements" then follow the prompt to relaunch you should find everything cleared up.

Is there any way to unfollow or hide the deleted accounts that I'm apparently still following on twitch?

I've found a solution that seems to work while still using dark mode. If you go to to flags using opera://flags then click on the drop down box next to "Auto dark mode for Web-Contents" and select "Enabled with selective inversion of non-image elements" then follow the prompt to relaunch you should find everything cleared up.