I have difficulty with the theme of FrameMarket

I want to change the css on the theme but I don't know how to do.

And I got a problem also. When a go on the settings of the theme and I choose a type of header. I click on save options. The type of the header return to nothing

parent theme : http://developpementpersonnel.ca/
child theme : http://boutique.developpementpersonnel.ca