I strongly agree. I use dark modes on almost all my other desktop applications where it is available, but after a few hours of using the dark mode in Slack my eyes got tired.
If they allowed customization of the main dark background color this would be remediable, but the only part of the theme that can be changed is the sidebar.
After literally years of promises, this is such a poor implementation that it's shocking. And I say this as someone who actually mostly likes Slack as a product.
Like the font weight is too high or something. I had to change back to Light mode.
I'm all about dark mode but not a fan here. They need to spend some time and make it more readable.