You just need to add some css to the bottom of your theme or child theme's css. I'm not using that theme, but you have to check out where it prefers to have your additions. If you have to add it on rather than having a separate file as many do, make a comment line indicating what you are adding below and remember to back it up before updating so you can re-code it afterward.
Remember that when you do this you may cause a few other things such as elements in your header or footer to get out of alignment. Here is a thread in another forum about one way to make this adjustment and some of the issues that arose with comments about how that was dealt with. It should be helpful to you.