Use CSS to change button color by class (ThingWorx 9.0)

I'm struggling with the CSS to change a button's color dynamically (by passing in different class names to the widget).  I'm able to get this to work:

#root_ptcsbutton-7:not([no-wc-style]).widget-content {background: #FF0000; !important}

But it's tied to the ID of the button.  I'd like to do it by class instead so it can be more generic.