As I understand you want to reskin myapplications.microsoft.com page for your organization. If you are looking to add a custom background page on myapps portal, then this is not possible as of now. Currently we do not have this feature in place.
However, you can do the branding only at tenant level through company branding tab. This will appear to users whenever they try to access any Azure services and when they get redirected to Azure authentication endpoint.
You can refer below article to configure company branding,
https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/customize-branding
If you are not looking to configure your organization's Azure Active Directory sign-in page and if you want to configure branding page on myapps.microsoft.com then I would suggest you post this idea at the Azure Feedback Portal, which is monitored by the product team for feature enhancements.
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