"Made with ❤️ by the Zen Team" in the footer
First noticed this mentioned in this Reddit thread, and it doesn't look like anyone has made an actual GitHub issue regarding it as of yet.
As shown in the above image, the contrast ratio for the text here is 1.82:1, rather than WCAG's recommended minimum of 4.5:1. The second screenshot was taken inside the accessibility pane in the Inspect menu from within the Zen browser. This only seems to be the issue in dark mode, where the footer has a lighter background colour.
"Welcome to a calmer internet" in the home page tagline
Also, upon checking the site myself after coming across that thread, I also noticed that the word "calmer" in the tagline on the home page (while in light mode) has contrast below WCAG standards (2.5, where I believe it should be at least 3 for larger text like this):
