var stylesheet, breite;

if (window.innerWidth)
{
  breite = window.innerWidth;
}
else
{
  breite = document.body.clientWidth;
}

if (breite < 500)
{
  stylesheet = 'screenpda.css'
}
else if (breite < 900)
{
  stylesheet = 'screensmall.css';
}
else if (breite < 1600)
{
  stylesheet = 'screennormal.css';
}
else // breite > 1600
{
  stylesheet = 'screenwide.css';
}

document.writeln('  <link rel="stylesheet" type="text/css" href="' + stylesheet + '" />');