There is a Citrix knowledge base article on how to apply the White Theme on a Citrix Access Gateway Enterprise Edition, right here: http://support.citrix.com/article/CTX123607
There is a Citrix knowledge base article on how to retain Custom Settings on a Citrix Access Gateway Enterprise webpage, right here: http://support.citrix.com/article/CTX123780
But the problem is when you do both (Apply White Theme and wish to retain your customization) there is none.
Problem is that both actions copy the content to /netscaler/ns_gui/vpn directory where the White Theme is copied last.
What you basically can do is edit the file /nsconfig/rc.netscaler to add the following rule: cp -r /var/vpn/vpn/* /netscaler/ns_gui/vpn. Make sure your customized content is copied to the /var/vpn/vpn directory.
So if you apply the White Theme and wish to retain the white settings the content of your /nsconfig/rc.netscaler should look like this:
tar -zxvf /var/white.gz -C /netscaler/ns_gui/vpn/images
cp -r /var/vpn/vpn/* /netscaler/ns_gui/vpn