Change color of one element on website

Hi, thanks in advance to anyone who even bothers to read my little cry for help.

So, I am trying to change the background color of just the first content box under the slider on this website I manage: http://www.latinophysiciansofca.org.

I think I need to create either an alert or a class in order to change one box but not the others. I don’t know exactly how or where to create a class/alert.

Could someone please show/tell me how I would go about doing this?

A bit more detail:
There are four content boxes located just beneath the slider. In order to highlight the information in the first box on the left, I would like to change the background color from the color white that it is now, to a different color, like red or blue.

Hope that’s clear.

Any help would be much appreciated!