Site different in every browser, and on different computers

My site looks different from my mac to another persons mac, and it looks different in different browsers. It also looks different on a computer, than on a mac. The size of the text, the font type, everything looks different. Do you know how I can change any of this to make it look the same in every browser, on every computer and mac?

Pretty please, I don’t know what to do.

The theme I am using is Grid Focus.