if(screen.width>1600){
    document.write('<link rel="stylesheet" href="styles/bg1920.css">');
}
else if(screen.width<=1600 && screen.width>1280){
    document.write('<link rel="stylesheet" href="styles/bg1600.css">');
}
else if(screen.width<=1280 && screen.width>1024){
    document.write('<link rel="stylesheet" href="styles/bg1280.css">');
}
else if(screen.width<=1024){
    document.write('<link rel="stylesheet" href="styles/bg1024.css">');
}