As you have added logo – it cant be exactly center aligned in header as per design. Because header is designed to have multiple sections. But instead you can add banner in header. Please go to Admin dashboard – Theme options – Header – Header Drag Drop section. Add just Banner element in Active elements section. Go to Banner options and upload the banner image. Once you upload you will have to add the css to center align the banner. I hope this is clear.
Just wanted to mention that getting assistance with small css/html changes comes under plus support. You can get more details here – http://cyberchimps.com/plus/. Let me know if you sign up.