Support System CSS styling changes

Good day

I need to change some colours using css for the Support System plugin to fit in better with my website. I specifically want to change the button colour for the button as indicated in the graphic below.

Using Firebug, I have made the changes and added the css to the theme's custom css area.. Unfortunately the changes are not showing.

I could make changes to the plugin's cs files, but these changes would have to be made every time the plugin is updated.

Any suggestions would be appreciated.