You can use the following page to create a custom theme for your app: https://ux.mendix.com/theme-creator.html
Also I would recommend to adjust only Sass files and not the CSS files as they are generated from Sass files. On the UX site from mendix (link above) you can also find information about editing the CSS.
Jeffrey's advice is good, and using your the theme creator will allow you to donwload a zip file that will update the look of your app. However, you probably want to go further than this. In the zip file that the theme creator produces is the complete folder structure of the Mendix theme folder for a complete styling package. Unzipping this into your Theme folder would give you the files you need to manipulate as your 'default' theme.
As Jeffrey recommends, don't edit the CSS directly but rather use the SASS files with a compiler to ensure your changes are saved and propagated. Here is a great link explaining the SASS framework:
Thanks guys. I'll try it and let you know if I am successful