Month View: Change color of past dates?

Hello, I'd like to lighten the color of past dates in Month View so the contrast from past to present/future dates is easier to see. Can you please guide me to the correct CSS? Also: I'm using AudioMan Pro 2.3.0. I see there's a 2.3.1 update available. If I update, will I lose customizations or additional CSS? Thanks, Steve Johnson https://www.stevejohnsonmusic.com