Terms & conditions popup offscreen and transparent background

Hi,

My popup box in checkout is going offscreen.
This is what I get in inspection:
<div id="modal-shop_terms_and_cond" style="width: 470px; margin-left: -235px; display: block;" tabindex="-1" class="modal hide fade show" aria-modal="true">
<div class="modal-header">
<button type="button" class="close novalidate" data-dismiss="modal">×</button>
<h3>Algemene Voorwaarden</h3>
</div>
<div class="modal-body"><iframe class="iframe" src="/checkout/checkout/termsandconditions/step-1/pos-3/tmpl-component" name="Algemene Voorwaarden" height="480" width="450"></iframe>
</div>
</div>

Where can I find the css for this? I would love to place it center screen and with white background so it is readable.
Also the box will not close, is this a jotcache or jch-optimize issue?
Thank you