How to edit the toggle button

Forget one:
I can′t get rid of the turquoise link color.
Any advice?

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: https://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;
}

I can′t get rid of the turquoise link color.

This should do the trick for changing the link colour within the body of posts and pages:

.entry-content a {
  color: #xxxxxx;
}

Thank you very much for your detailled instructions, Kathryn.
It will help me as a CSS-doofus to better understand the theme.
For now I′m happy with the toggle button label. Your snippet works fine as long the header image option is unticked.

Keep up the good work.

Tom

My pleasure Tom – good luck with your site!