Unavailable padding value for Google custom search bar

Hello,

For setting padding value of form of CSS for my google custom search bar,
it's unavailable in drupal default theme when I set it into .form {padding:0}.

however, the real value is still in about 5px, so the search bar was dislocated.

can anyone tell me how i can fix it? many thanks..

p/s:
I added a background colour for finding out the issue conveniently.

http://drupaltaiwan.org/files/12345.jpg
http://drupaltaiwan.org/files/54321.jpg

thx.