You have two choices.

1.) You just use FTP (if the server isn't local) and overwrite the current login.ascx file (you'll have to do this everytime you upgrade your DNN core) or

2.) Create your own authentication provider using the current DNN code as a base/template. Then you can use the Authentication package uploaded (under Host->Host Settings) to upload the new authentication package to your site.

I have seen some re-designed login views- I have seen a great micro login module - but they were not for sale by the developer since the company it  was done for owned the source code.

But the solution there was to create a PA and apply it as a third party module to a page.

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