The text color is almost black - 34,34,34. It uses a webfont called Minion Pro https://typekit.com/fonts/minion which isn't especially thin. The fallbacks in the font stack are just serif and sans-serif.

That is not what I see in my Firefox:

https://imgur.com/a/v3Oo1uy

It is probably some kind of weird configuration in my computer though.

You hit an arrow key. This page has weird scripting that tries to let you navigate with arrow keys, but I noticed it ends up skipping most of the text that way.

I'm guessing they used this document as their 'slides' while presenting. The text is what they'd be saying, while the arrow keys hide the script, and flip between annotated graphics.

That said, the keyboard navigation is pretty much useless outside of that context.

I think it has something to do with the scroll hijacking. The text is almost white if it thinks you haven't "scrolled" to it yet.

I have tried to scroll, and move around but the contrast does not change.