The default login.html page contains this line:
so it is using the login.css in the ui/theme-default location. This is one of the default themes that ships with Mendix and will be overwritten each time you restart the application from the Modeler.
To avoid your changes being overwritten, you need to include the login.html file in your bespoke theme and use a css file in a different location that will not be overwritten.
the layout.css can be used to alter the login layer of the index.html file in your theme directory. You probably need to alter the login.css which can be used to change login.html.
Check your browser which file is loaded.