@urlyman I'd say about 90% of what most overlays do is CSS adjustments. But I'd only spent about 30 seconds looking when I replied to your post. On a closer look, it looks like this has a narrower range of features than a lot of the overlays, and some of the most problematic are missing, so that's a mildly positive thing.
In my opinion, though, this is still not very useful. This functionality should (and does) live in the operating system, browser or a browser extension. It's a red flag, in my mind, when a website owner thinks it's their responsibility to provide these options, and that they think a third party script is the way to do it.
In this case, it feels like this is mostly performative. For example, if you increase the text size, the items on the homepage get truncated and some become unusable, so it seems that the website owner/designer/etc hasn't even tested these functions themselves.