You should be able to style that either in the modeler if you are using an anonymous users enabled app or by directly changing it in the login.html file in your theme folder if you’re not using anonymous users. I’m assuming you’re using the latter, so this might be a case of finding which background image is used in your login.html and replacing it.
Hope this helps.
Yes it is. If you open your project in file explorer you can go into the theme folder and see a login.html file. You can make changes to this file.
Also there is a learning path section about this.