Where is the code stored to change the grey bars and grey shadows in the boxes to another colour that more suites the site?

Hi,

The codes are stored in the Global CSS file, which is right here:

ROOT/templates/<yourjoomlatemplate>/html/com_virtuemart/assets/css/flexibleVM2Global.css

and everything is stored here. We have used CSS3 Gradient backgrounds while creating those buttons, so it won't be easy for you with just by changing HEX color codes. You can use this website: www.colorzilla.com/gradient-editor/
to generate your gradient backgrounds, then just copy-paste them to proper CSS rule in the Global CSS file.

Cheers.