From inspection the responsiveness is due to the CSS (rather than browser detection or serving a different theme based on the UA of the request).
If you inspect the page source you should spot the code that is controlling the responsiveness:
<script type="text/javascript">
var ADAPT_CONFIG = {
path: "http://www.trend-store.nl/skin/frontend/default/galamrhandsome_black/css/",
dynamic: true,
range: [
'0px to 760px = mobile.css',
'760px to 980px = 720.css',
'980px to 1200px = 960.css',
'1200px = 1200.css'
]
};
</script>
This variable ADAPT_CONFIG
is passed into a function in this javascript file
So I think to remove the responsiveness you will need to hack this theme a little bit - and there may be unpredictable consequences. Maybe the theme author would make a small change for you.
I think there are two ways to hack your theme by replacing the code snippet above, or just adding one of these scripts immediately after the snippet above:
<script type="text/javascript">
var ADAPT_CONFIG = false;
</script>
or
<script type="text/javascript">
var ADAPT_CONFIG = {
path: "http://www.trend-store.nl/skin/frontend/default/galamrhandsome_black/css/",
dynamic: true,
range: [
'0px to 760px = 960.css',
'760px to 980px = 960.css',
'980px to 1200px = 960.css',
'1200px = 960.css'
]
};
</script>
You will need to search in the theme files to find out which file to edit. Probably [path to template]/page/template/html/head.phtml
I recommend a lot of testing. There may be more edits required and apparently you can find more about this responsiveness method here http://adapt.960.gs
**EDIT following comment below
It looks like you also need to change the veiwport meta tag from
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
to
<meta name="viewport" content="width=960, initial-scale=1.0"/>
assuming 960px is the full page width, but we may be opening a can of worms here.