Hello Esther,
The menu bar was designed that way. I can assist you on bringing it over the top and having it stretch end-to-end if that is what you want.
This requires some coding which I am going to assist you with.
Open your index.php file and copy the following block right bellow the opening <body> tag.
  <div id="menuOuterWrapper">
    <div id="menuWrapper">
    <div id="menuToggler"><?php echo JText::_('MENU'); ?></div>
      <div id="menu">
        <?php if($this->countModules('nuMagazine_menu')): ?>
        <jdoc:include type="modules" name="nuMagazine_menu" style="nu" />
        <div class="clr"></div>
        <?php endif; ?>
      </div>
    </div>
  </div>
The rest is css. In your custom.css file paste this block.
#menuOuterWrapper { left: 0; min-width: 100%; height: auto; }
#menuWrapper { margin: 0 auto; max-width: 1500px;  }
#menu ul li a { width: auto; padding: 10px 20px 8px; }
#menu ul li a:hover { width: auto; }
@media only screen and (max-width: 569px),
  only screen and (-webkit-min-device-pixel-ratio: 1.5) and (max-width : 569px),
  only screen and (-o-min-device-pixel-ratio: 3/2) and (max-width : 569px),
  only screen and (min-device-pixel-ratio: 1.5) and (max-width : 569px) {
#menuToggler { text-align: left; }
}