Hi,
There are no editors that will take someone elses templates and let you mess around with them. Its all manual editing I'm afraid.
If you want to design your own then there are programs like TemplateToaster that does virtuemart as well but it isn't perfect and I still only use it as a starting point for some projects
There are also template designers that install into Joomla but they don't do the virtuemart bit only the Joomla bit.
For CSS there is a useful editor by Skybound (
http://www.skybound.ca/) called Stylizer. It shows you the changes in real time. Bit of a learning curve but it means you can create your own style sheet for your tweeks and you can do a lot of layout changes just with the CSS on most templates. Its also a great program for learning about CSS without trashing your site as the changes are done in the program, on the fly to the page you are on, and then saved to a CSS file to upload.
You can buy templates but a lot of sites are now on a subscription basis so can turn out a tad expensive for the better templates that have more features (such as
https://www.virtueplanet.com/joomla-templates/).
Sorry there's no drag and drop solution. One day maybe but not anytime soon.
Regards
D