That topic was just an example. It is demonstrating how to use "Firebug" plugin to change the "add to cart" button style.
in your case, the CSS file is:
for VM3.0
/templates/YOUR_TEMPLATE/html/com_virtuemart/assets/css/flexible.css
for VM2.6:
/templates/YOUR_TEMPLATE/html/com_virtuemart/assets/css/flexibleVM2Global.css
but Firebug will tell you this already.
Without using Firebug (or Chrome's Developer Tools) you can't make any CSS changes. Because CSS files have thousands of lines and there are many CSS files for one website. Those plugins will tell you "what css file and what css lines" you need to make edits.
After you figure out the file name and the line number, all you need to do is open that file with a text editor (i.e: Sublime), find the proper line and change the background rule. FYI: You can generate CSS3 rules for gradient background colors through this website:
www.colorzilla.com/gradient-editor/
If you are newbie to CSS and newbie to web design. I would strongly recommend you to see this topic:
www.flexiblewebdesign.com/forum-virtuemart-2-template-puremart-support/342-using-firebug-for-customization-or-troubleshooting
and watch the 2 video tutorials. (PART ONE and PART TWO). There are also dozens of other videos on YouTube as well.
Using Firebug is the "easiest" and the "most professional" way for any CSS modifications.
Regards.
Support