On Mendix 8 you can use nanoflow Action to start the bar code scanner, that way you can leverage on standard buttons and do more awesome workflow.
When your app is running in the browser you can inspect the buttons by right clicking them and then selecting inspect. Is the css you have given the buttons being applied correctly or is it being overwritten by an other element?
I agree with Edo. You can also use Calypso and check on the fly when you change the css of the widget.
Thanks for the answers!
I’ve inspected the barcode scanner buttons as Edo advised and none of my css attributes were applied to them at all. I toyed around a bit with them in DevTools just to see if everything worked correctly, but the final solution was what Andries suggested.