BidJS page elements can be styled using CSS. We have found that one of the best ways to figure out what changes to make and to try them out is by going to our demo site (or your own site if the software is already installed), right-click on an element (like a button) and click 'Inspect', then hit the '+' button and enter temporary CSS code to see what it looks like on your site:
You can then copy the content from there into your CSS sheet and the changes should work as expected, e.g.
button.lot__action.lot__action--bid.btn.btn-primary.btn-block {
background-color: purple;
}