Change font, reduce width and set image for header

Hi, I'm still new to wordpress and customizing, please bear with me for the following questions. 1. How do I change the font to Arial that we can see in the full demo of catch responsive? 2. How do I reduce the width of the whole blog to something like 80% or 70%? I have tried suggestions provided in many other topics but nothing changed. 3. How do I include an image in the header to replace the site title and tagline? Can I set this image in the center? Note that I'm using a child theme for modifications. What do I need to include in Custom CSS? Here's my website URL in case you need it: http://www.acefiction.com/