Doing a Joomla Template Override is easy.

Open the view folder you want to change, THEN, the tmpl folder.
COPY the "default.php"
THEN< follow the steps below.
In your JOOMLA template folder. You create a folder called  "html" then a folder called "com_virtuemart" in it.
Then for each view you want to override. Create a folder for it.
Then place your modified "default.php" an that views folder.
So , to override the product details page. You create YOUR_TEMPLATE/html/com_virtuemart/productdetails/default.php

Doing a Joomla Template override guarantees the view will use your modified default.php

FOR changing the css. What you do is copy the virtuemart css located in
com_virtuemart/assets/css/vmsite-ltr.css

THEN, add that to your joomla template css file. (or call it seperately, AFTER you put a copy in your Joomla template foler)
THEN, go to vmart configuration, and turn off virtuemart css