Visual Form Builder CSS cannot change

I’m using Visual Form Builder.
I want to give my forms the same font etc as my website.

I followed the instructions on the support site: In my theme folder, I created a folder ‘css’ and added ‘vfb-custom-css.css’. Did that via Filezilla.

* For some reason, if I follow the instructions, the created file and folder don’t show up in the editor in WordPress. My edits also don’t show in the form (my edits were simple things like other font, other background color etc. – based on the tutorial code).

* I tried putting the file vfb-custom-css in the functions folder (changed the code accordingly) but that doesn’t work.
* Then I tried putting it in the / folder of my theme. Now it shows up in the editor, but still the changes I made, don’t show.

To make the changes to font etc, I just copied the code from the support site and changed the values.

My questions:
1. Did I do something wrong?
2. If I instead overwrite the visual-form-builder.css, will I loose my changes when there’s an upgrade to the plugin?

Thanks very much!