changing font colours in checkout and shop

I am a newbie so some detailed help would be appreciated.
I have searched documentation but to no avail – maybe I am just not understanding properly so some gentle (or not so gentle if required) guidence please.

I want to change the font colours on the store and checkout pages.
I have tried to add some css code to the text box but it has no effect – what code do i need?
And is this the right place to put it??

Thanks in advance
Dereck Gray
Dunedin, NZ