This css code is very handy. It can reduce gap between header and body . That is if the the below space of the header is more and you want to reduce this gap then this css code is very handy.
Go to Design > Template design > Advance > then put this code in the css text box.
The gap is controlled by varying margin-top:-5px values. -5 can be changed to any value from 0 to -25 for better result. This depends on the template of your blog.