Hi,
.chosen() is a 3rd plugin that comes with Virtuemart Core. It is really hard to customize it. I would recommend you not to touch the CSS file of .chosen() plugin. (to lear more about .chosen(), google it)
But instead, i would recommend you to disable the .chosen() and make those dropdown-select box as "regular dropdown select box", Then you can use CSS to customize the appearance of them very easily (it would be a much easier approach)
To disable .chosen(),
1-navigate to here:
VirtueMart Backend >> Virtuemart Configuration >> Templates [TAB]
2- Uncheck this checkbox: Use jQuery chosen for dropdowns in FE
Regards.