If you want to change the styling, use a custom css file (that is included in your index.html) and overwrite the css.
What exactly is it you want to achieve?
If you mean mxui.css, that's the default stylesheet for Mendix.
If you want to change your styles, look in your project's styles folder (Default: DocumentsMendix\<project name="">\Theme\css) for a file named mendix-custom.css, and add your styles there. Or you can add another custom css file by editing the index.html file in your theme folder.