Change site designin secure zone

I was wanting to place a clients ordering system inside my site as a secure zone but need to change the colour scheme inside that secure zone.

Things like page background colour , menu colours etc…

Is that some thing easy to do?

Yes, there are various ways to do this, but one basic example is that you could load another CSS stylesheet when someone is logged in.
So in your site <head>:

{% if request.is_logged == true %}
<link rel="stylesheet" href="/css/overriding-css.css">
{% endif %}

More info on the request object and member login details here: