Hi Manuel,
First of all, i would like to make things clear.
DarkMArt is a joomla template, but it supports Virtuemart stylization.
You can't take apart the Virtuemart stylization from DarkMArt and apply it to your existing Joomla Template. This just won't work. We have a different product for this purpose, which is called PureMart and it is a Virtuemart Theme (not Joomla template)
(PureMArt lets you keep your existing joomla template and style the Virtuemart related-pages only)
If you want to apply the DarkMart to your website and you are starting everything from scratch, you should use the QuickStart package at first. So you will get EXACTLY same layout as you see in our DEMO page, then you would start customizing things. Such as; changing menu items, adding your contents and Virtuemart products etc..
One-by One method is only for "updating" the specific extensions. For example, there is a new version for dropdown shopping cart module (that comes with DarkMart), then you should follow the One-By One method and install only the dropdown shopping cart module. Because "QuickStart" method will reset everything back to stock.
So, if we get back to your issue, if you would like to use DarkMart, you need to give up from ms-exterra (which is your current joomla template). You can't use both of them at the same time. Actually you can. You can choose specific menu item to load DarkMart for those menu items, but you would lose the harmony, the layout would't be the same in different sections of your website which is not a professional way. I would suggest you to keep the same design and layout in all sections of your websites. Otherwise, your visitor would think they were redirected to some another website when the layout is changed.
If you insist on using
ms-exterra joomla template, then you can't use DarkMart, so you should be using PureMart instead. (which is a Virtuemrt Theme and it can be integrated to any joomla template).
You should decide which one you are going to go with.
I can replace your product (DarkMart) with PureMart free of charge. You can take a look at PureMart through this link:
www.flexiblewebdesign.com/Virtuemart-Templates/Virtuemart-2-template-PureMart
Then let me know.
Regards.
Support & Development.