You must not change anything in the deployment folder, since this is only reflected for your machine and overwritten when you run the project again.
If you want to change the style of the widget, best to do is to give it a class and add it to your own scss (see for example calypso https://docs.mendix.com/howto/front-end/calypso).
Hope this helps!
Your css changes should be located in (. /theme/styles/web/widget/widgets.css) something like. Try to change your styles in this path so it will reflect in UI. And (. /deployment/web/widget/widgets.css) this path is dedicated to the build changes in your project structure.