@Anonymous For this purpose, you would need to use your own HTML and CSS file and upload the files to a location where these files can be accessed publicly. I have uploaded my HTML and CSS file to Azure blob storage. For your reference, you can find the files here: unified.html and global.css
If you are using built-in user flow, you need to Navigate to your user flow > page layouts > set Use custom page content to Yes and set Custom page URI to the url of your html file.
If you are using custom policy, add below XML code to either TrustFrameworkExtensions or your RP (signup/signin) file. Where the value of LoadUri must be the URL of your custom HTML file.
<BuildingBlocks>
<ContentDefinitions>
<ContentDefinition Id="api.signuporsignin">
<LoadUri>https://moviesdiag132.blob.core.windows.net/b2c/unified.html</LoadUri>
</ContentDefinition>
</ContentDefinitions>
</BuildingBlocks>
-----------------------------------------------------------------------------------------------------------
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.