@Caspar Clemens:
For 1 and 2. you can just add the following css in “Appearance => Theme Options => Custom CSS” box.
#header-content,
#branding .logo-wrap {
margin: 0;
}
For 3, it’s bit tricky and I am not sure it will work on not. As this theme is not designed to make it 100% width menu. So, try to add the following css in “Appearance => Theme Options => Custom CSS” box.
@media screen and (min-width: 1001px)
.wrapper { width: 978px; }
#access ul li a { padding: 0 17px; }
#access ul li.page-item-4034 a { padding: 0 13px 0 14px; }
#access ul li.page-item-4034 { border-right: none; }
}