Want to change mobile device CSS

Hey there, I'm using the new Upfront theme and really liking the interface. I've got the desktop version of my homepage pretty much sorted but I am struggling with setting things up for the various mobile sizes.

For each of the various viewport sizes provided in the Upfront Responsive module I have updated things so that they are the way that I want them but when I view the site on a responsive tester at sizes different to 570px and 315px

@1024 H1 remains black instead of white
@640 H1 remains black instead of white + icons are off centre to the left by a few px

I've found the respective CSS for some viewports sits in admin-ajax.php but not sure how I should set these up.

Also whenever I go into the responsive editor my saved changes never seem to save at the 315px size. I have to resize the icons each time and then rearrange the order because the heading always slips to the bottom of the second region.

Any help you can provide will be greatly appreciated.

Cheers... Brett