I'm pretty sure that they're referring to the mods that change the shadow values for strand hair. The ones that I know of are these:
These mods sort of solve the problem to varying degrees of success... I'll add screenshots and additional details, etc., momentarily. In the process of gathering info, graphics drivers crashed (another fun issue that so far only happens with this game), so I have to restart, and I'm going to submit this post to avoid having to redo the links, etc.
Updating to add screenshots, etc. You can see that there's some improvement with Strand Hair Shadow Tweaks enabled. But it causes all hair to look more flat in lower lighting. And light hair still ends up looking dark in various lighting situations.
The Better White Hair mod increases reflective values too much, and the same hair looks bright white in most light, and glows in all of the same locations except for the character screen. It doesn't cause other hair to look flat (e.g., Harding and Bellara), but I wouldn't say that it fixes the problem for Rook's light hair at all.
The Lighter Hairstyles mod doesn't affect the same hairstyle, but with a similar hairstyle (the looser, higher-set bun), the effects were similar to Strand Hair Shadow Tweaks.