Hi Jazib
If you go to:
Documents > Mendix > [Your Project] > Theme>
You can then make any changes you want to “login.html” page or replace it entirely with your own html page. If you have any CSS or JavaScript you want, just create folders for them named respectively inside the Theme folder and include them in the header (for CSS) and at the end (for JavaScript) of your new html page.
However, please make sure to keep the <form> parts and the following lines as these holds the username and password input intact:
<script src="js/login_i18n.js?{{cachebust}}"></script>
<script src="js/login.js?{{cachebust}}"></script>