Below content is applicable to both Atomic and AX2:
The customisation for the payment pages is stored as part of the organisation details in APEXX in form of a .css file. If you require differently presented payment pages per brand (or simply wish to retain some flexibility around your styling options), you can send the transaction_css_template attribute in the hostedPaymentPage request.
Complete attached .css file with the styling options for your organisation(s) and forward it to your APEXX implementation manager
Send the .css data as part of the payment request - sample JSON file attached. This feature needs to be switched on by your Implementation Manager - if the styling options don't appear as expected on the first attempt, just ask for the use of individual templates to be activated.