Need to Change Andreas09

I am using Andreas09 I thought that the top bar was alittle big so I went into the graphic and made it 20 px smaller, however in doing that I need to move the div id “mainmenu” up the 20 px so that everything lines up again. so is there an easy way to do this. I am a beginner, I have been playing with WordPress now for about three days. One thing that I did try was the padding in the CSS and that did nothing. If you need to see a graphic https://homepage.mac.com/jeffandmandy/testwork/testpicture.png. I made the boarder visable to see what is happening easier. I also want to make the “mainmenu” table thinner a bit so if anyone knows that. Thank you very much.