How to use CSS to show a file upload field based on a user choice selection?

Hello i would like to add a custom css that if a customer selects one section for a service e.g content writing that they in turn need to attach a file for the service picked

For this requirement, you do not need CSS, but it can be achieved using conditions. Here is related guide: How-to-Show-or-Hide-Fields-Base-on-User-s-Answer 

Here is an example:

Is this how you want to implement? Please review and update us.