First off, no need to post multiple times the same topics in the forums (I'm talking about the two topics you posted in the C3 forums, now all duplicates are removed).
Once is enough and will gather the required information.
Project files are files embedded within your project. You can check this tutorial - https://www.scirra.com/tutorials/328/using-project-files-in-construct-2 - to have an exact idea of what they are and what they do. The tutorial is for C2 but still applies to Construct 3.
Now it seems that what you are looking to do is actually different. You are willing to modify the CSS display of a button.
That has nothing to do with project files. Or at least not just as adding a css file.
As you can see in the button manual article (still from C2 but still applies currently until C3 has its own documentation) the button has a "Set CSS style" action.
Use this action to apply the CSS style you want.
You can possibly still use project file, but the thing is you would read the content of the file and apply it to the various elements you want, using the Set CSS style as you go.
We're now entering text manipulation you can read about and find some examples in the How do I FAQ for C2.