Gray boxes around the letters of the menu

How can I remove the gray boxes around the letters of the menu?

Please post your forum's link.

The image is broken, hence it shows a border which indicates that it's showing the alt texts. Do you have the following option enabled ?
Admin Panel > Display > Headers and Navigation > Display only images in the navbar

If so, change it to "no."

I selected "Yes" from the Admin Panel> Display> Headers and Navigation> Display only images in the navbar, because I want to remove completely (and for administrators) the buttons: «calendar», «faq», «search»,«memberlist»,«groups»,«profile» and «inbox». Is there any other way to remove these buttons from the menu?

Of course, you can use CSS.

Code:

a.mainmenu[href="/calendar"], a.mainmenu[href="/faq"], a.mainmenu[href="/search"], a.mainmenu[href="/memberlist"], a.mainmenu[href="/groups"], a.mainmenu[href="/profile?mode=editprofile"], a.mainmenu[href="/privmsg?folder=inbox"] {
  display:none;
}

It goes into Display > Colors > CSS stylesheet

Thanks o lot!!!