Writes the head part for the generated document,
           with the given title and CSS

def write_head(self, title, css_path, default_css):
        """Writes the head part for the generated document,
           with the given title and CSS
        """
        self.title = title
        self.write(
            '''<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>{title}</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link id="style" href="{rel_css}/docs.dark.css" rel="stylesheet">
    <script>
    document.getElementById("style").href = "{rel_css}/docs."
        + (localStorage.getItem("theme") || "{def_css}")
        + ".css";
    </script>
    <link href="https://fonts.googleapis.com/css?family=Nunito|Source+Code+Pro"
          rel="stylesheet">
</head>
<body>
<div id="main_div">''',
            title=title,
            rel_css=self._rel(css_path),
            def_css=default_css
        )