Make it active is not really a trivial thing.
The status is defined by the "action", without a new action it's basically very difficult to make a button active.
The only way that comes to my mind, is to let specify an action and a subaction on which it will be selected as active, though, even that is an incomplete solution because there may be situations it won't work.
No good answer here, sorry.
And this is more a problem with the way Elk highlights the button than the menu editor.
Something specific for that may be implemented in the menu editor, but at the moment, due to the way Elk handles icons in mobile view, you'd have to:
1) find the id of the button,
2) add a bit of css along the lines of:
#id_of_the_button .linklevel1:before {
font-family: 'FontAwesome';
border: 3px solid #666;
padding: 1px;
margin: 2px;
height: 1.15em;
width: 1.15em;
text-align: center;
display: inline-block;
border-radius: 4px;
line-height: 1.2em;
font-size: 2em;
color: #666;
content: 'the-code-of-the-fontawesome-icon-you-want';
}
For example, for your home page, it could be:
#button_f6f31cebde020d86add72bea0e662341-1468822676 .linklevel1:before {
font-family: 'FontAwesome';
border: 3px solid #666;
padding: 1px;
margin: 2px;
height: 1.15em;
width: 1.15em;
text-align: center;
display: inline-block;
border-radius: 4px;
line-height: 1.2em;
font-size: 2em;
color: #666;
content: '\f015';
}
while, for the Entwurfe button:
#button_6935862f74f620598b6b2549c202aafa-1468821375 .linklevel1:before {
font-family: 'FontAwesome';
border: 3px solid #666;
padding: 1px;
margin: 2px;
height: 1.15em;
width: 1.15em;
text-align: center;
display: inline-block;
border-radius: 4px;
line-height: 1.2em;
font-size: 2em;
color: #666;
content: '\f040';
}