You can add an additional CSS stylesheet to your vBCloud site by using Replacement variables.

First, create a file called additional.css to put your css in. Upload it to some site and grab the URL - http://www.site.com/additional.css

Go to Admincp > Styles & Templates > Replacement Variable Manager.

For the Search for Text textarea, enter:
HTML Code:
css_additional.css" />
And for the Replace With Text textarea, enter:
HTML Code:
css_additional.css" />
<link rel="stylesheet" type="text/css" href="http://www.site.com/additional.css" />
Save. That's it. Edit your additional.css file and make sure to replace the original whenever you make changes.