Can you try inserting the below piece of code into custom/custom-functions.php file and see if that works for you?
$prefix = mo_get_prefix();
add_action("{$prefix}_start_header", 'mo_display_button', 9);
function mo_display_button() {
$itunes_url = 'http://itunes.apple.com/';
$google_play_url = 'http://play.google.com/';
echo '';
}
and the Custom CSS –
/* Hide the existing get app button */
#header .button.get-app { display: none; }
.app-download-buttons img { margin: 15px 10px 0 0; max-width: 140px;}
.app-download-buttons { display: inline-block; width: 300px; float: right;}
.play-button img { border: 1px solid #aaa;}
.itunes-button img { border: 1px solid #252525; border-radius: 4px;}
#header.sticky .app-download-buttons img { margin-top: 5px; margin-bottom: 5px; max-width: 140px; }
/* Mobile layout */
@media screen and (max-width: 767px) {
.app-download-buttons img {
margin-top:0;
max-width: 135px;
margin-right: 5px;
}
.app-download-buttons {
margin: 0 auto;
width: 280px;
float: none;
}
}