How do I change this color on this menu with css?

As you can see from the screenshot, the color that indicates which option I’m currently choosing (“Conservation of Weak Isospin”) is too similar to its surrounding color and isn’t stand out enough. It always confuses me and I have trouble with this multiple times. I believe this is because I’m using CSS from GitHub. Does someone know how to change this color? The CSS I’m using is very long and I have no idea which one changes this color. I want to change it to a red color like “#d6555f”, and then make the text white to stand out in the red. How should I do this?