I don't believe Renpy has a class that directly affects contrast. But the list of predefined matrixes can be found here. https://www.renpy.org/doc/html/matrixcolor.html#built-in-colormatrix-subclasses
As for actually modifying it, you'll be doing that near the bottom of the 00auto-hightlight.rpy. Specifically around lines 186-192. You'll also likely want to add a change amount a bit higher with all the rest of them around 123-125. If you have any other questions I can answer, feel free to ask!
Oh! It would seem there is a ContrastMatrix that isn't mentioned in the matrixcolor documentation. So yeah, using ContrastMatrix should work I believe. Here's the source code to prove it exists.