Arh... my CSS sucks.
Okay, let's change it a little bit, I didn't know content was fairly limited.
.i-board-off::before {
background: url("https://www.lars-mikkelsen.de/images/forum/icon-off.png");
display: block;
clear: both;
content: "";
background-size: 100%;
width: 100%;
height: 100%;
background-repeat: no-repeat;
}
#posting_icons .i-board-off::before {
width: 1.5em;
height: 1.5em;
}
And the mobile part should not be necessary... unless I'm mistaken. xD