Removing a gap from home page

If your theme has a built-in custom CSS option, use it; otherwise, get a custom CSS plugin and put:

.site-content {
  margin-top: 10px;
}

Thank you Stephen
It worked Strongly
can I use this code in another themes too ?

Probably not. Not all themes use the same selectors for the main content area. You might want to learn how to use a web inspecting tool like Firebug.