Bad news for you: that login page is a bit "funnier" to customize ...

Have a look at https://gianniceresa.com/2017/10/odc-appreciation-day-customize-style-of-lightweight-login-and-vadv/  to better understand why.

This thing is an independent deployment in weblogic (as you can see from the URL) and it hasn't an official documented way to customize it as it's a JET-like kind of page.

You will have to edit the EAR file and redeploy it. As far as I know not really a supported way but I didn't see alternatives around.