I am a global admin in my tenant and I recently installed SharePoint Designer. I tried to open the admin center (https://mytenant-admin.sharepoint.com) in SharePoint Designer but I got an error that said I was unauthorized. I looked in to this and found that it was because custom scripting was disabled. However, I have received errors trying to enable it on the admin center temporarily (This operation is not supported for this resource) and I am aware of the security issues, so I hesitate to do so. Is it possible to access the admin center in SharePoint Designer?
As a Short Answer: No, it's not possible!
Due to SharePoint Online is a part of M365 that is a software as a service (saas), and generally, in SAAS as a multi-tenant model, you are not allowed to customize the resources that you are not own like Admin Center, however, you are only allowed to customize what you are able to create. in case, it's permitted to do that!
As per the above rule, you can't create your own admin center, so you are not allowed to customize it, meanwhile, you can create your own SharePoint site and customize it based on the permitted options.