how to change the color of the menu is now gray.

_{The little menu under each page, it's gray now.}

_{Wich (css) file i must change to make it red or another color?}

Thank you.

A screenshot would be useful to help us understand exactly what you mean.

I suggest that you acquaint yourself with Firebug in Firefox or inspect element in Chrome - this will be of great assistance to you.

Find in  CSS file and change 

div.extra_top_menu table.fixed_menu { 
    background-color: rgba(to what ever colour you want, 0.8);
}

Thank you :-)