Hi @haj! It sounds like you're looking for something like our new beta feature mentioned here: Announcing: custom product pages!
Is your Retool instance on the Business plan? If so, you can join that feature's Private Beta to test it out and share feedback with our team as they perfect its functionality. To do so, reach out via either direct message here on the forum to @antonybello or or reach out in the custom-product-pages-beta channel in our Discord server. (If you aren't a member, join here)
More generally, there are branding options available on Business or Enterprise plans as well, but the new feature mentioned above is aimed at fully offering a custom-branded login page. We'd welcome your early feedback on this feature if you're open to joining the beta!