Hi Isabel, you are changing CSS files in the deployment directory which get overwritten when deploying your application.
You want to add custom styling using the right css classes to target the classes used by the widget. Check the following document on how to add custom styling in Mendix:
Note that there is a difference in where to add your custom styling between Mendix 8 and Mendix 9. I believe that you are in Mendix 8. If that is so you can import your custom style files in the theme/styles/web/sass/app/_custom.scss file.
Here is what I did to edit to the _custom.sccs file. You will find my addition at the very bottom of the CSS file. Although the code saves, no visuals are changed in the app.