Assume people use reader mode
Don’t assume everyone uses Chrome just because you do.
I recently confessed that I fully assume everyone uses Safari. I know that a lot of people don’t. But I do, and I feel that I pay for it, so I should use it. I don’t regularly test this website in any other browsers, but I do develop against open web standards (most of the time).
However, one assumption we should agree on is that people will use reader mode. They should. You should. I use reader mode on my own website. I will use reader mode on your website. It’s worth spending a little less time getting your color scheme just right and a little more time getting your markup just right.
No amount of fancy CSS is worth it if I miss half your article because reader mode doesn’t work correctly. I would literally never know.
Use reader mode. Assume people will use reader mode.