Thanks for pointing out the bug! The mind gauge/sanity mechanic didn't get enough play testing put into it, being in the dark doesn't make it go down faster; Instead it being low makes it decrease faster, and you can rest at anytime to recover [R] which makes you lose time. I played a game this jam that had a similar mechanic called "The Paperchase" which had an inhaler instead of resting, and they had a timer on it with the control key labeled which made it more clear when you should and how to use it, which I think our game could have used.
Also yeah, you can use RichTextLabels in Godot and by enabling BBCodes you can add all kinds of effects to highlight text, it is super cool. Check out the docs for some cool examples: https://docs.godotengine.org/en/stable/tutorials/ui/bbcode_in_richtextlabel.html