Their product are the bits they ship to you and the code they run on their servers.
You're modifying a part of their product.
Since you can't modify their product, you also obviously shouldn't be able to use any web browser to view slack because those browsers take those bytes they ship you and interpret them in the context of the browser windowing system (its userChrome.css for example)... that's also clearly modifying the experience they delivered to you.
You also shouldn't be able to use a computer monitor that has color settings different from the web designers working at slack. If you accidentally have a monitor with slightly less blue than theirs, well, you just modified the slack UI to be a little less blue. Obviously a ToS violation and you shouldn't use them at all.
God forbid if you zoom in on the page to modify their product's resolution/size.
Jerk.
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