How to edit the toggle button

How can I customize it? For example put a label on it´s side?

You could definitely experiment with some custom CSS, for example:

.toggle:before {
	content: "Menu";
	color: #777777;
}

More about the content property: http://css-tricks.com/css-content/

in which line of the stylesheet is the light grey color of the header and the toggle menu defined?

By using a browser inspector, you’ll see that the menu toggle icon has the class toggle and the site title has the class site-title in an h1 tag.

You’ll find the toggle colours defined starting at line 1048:

.toggle .menu-toggle-image {
  cursor: pointer;
  fill: #777777;
  width: 24px;
  height: 24px;
  -moz-transition: all 0.3s ease-in-out;
  -o-transition: all 0.3s ease-in-out;
  -webkit-transition: all 0.3s ease-in-out;
  transition: all 0.3s ease-in-out;
}
.toggle:hover, .toggle:focus, .toggle:active {
  background-color: transparent;
  box-shadow: none;
}
.toggle:hover .menu-toggle-image, .toggle:focus .menu-toggle-image, .toggle:active .menu-toggle-image {
  fill: #333333;
  -moz-transition: all 0.3s ease-in-out;
  -o-transition: all 0.3s ease-in-out;
  -webkit-transition: all 0.3s ease-in-out;
  transition: all 0.3s ease-in-out;
}

The site title colour is defined starting at line 983:

.site-title a {
  color: #777777;
  text-decoration: none;
}