This is called company branding and yes it is customisable.
When users authenticate into your corporate intranet or web-based applications, Azure Active Directory (Azure AD) provides the identity and access management (IAM) service. You can add company branding that applies to all these sign-in experiences to create a consistent experience for your users.
This article covers how to customize the company branding for sign-in experiences for your users.
Hope this resolves your Query !!
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