Good resource for learning how to read CSS?

Hello,

Can anybody recommend a good resource for learning how to read CSS? Most of the answers here seem to be very obvious upon retrospect if one knows where to look. For example, I asked about removing the featured image function in the Pilcrow theme and was pointed to the specific section of code to remove. I feel like I could have done this on my own if I had known what I was looking at.

Thanks for any advice!