To customise the look, naming, labeling, and accessibility of Web Content Accessibility Guidelines (WCAG), all of the website pages outside of custom checkout fields can be made to be WCAG compliant and are designed by the merchant. You could support WCAG 2.0 AAA level in which Bambora does not impose any restrictions on your page.
As for the styling, you will need to supply the CSS as per what’s supported in https://dev-apac.bambora.com/checkout/guides/custom-checkout/references (customercheckout.create → options.style section)