Generally speaking, it is not a good idea to update the plugin css file, as any updates would overwrite your changes.
Many themes come with custom css panels where you can enter your own code, but, if you don’t know how to enqueue you own custom css file (or any other custom asset file), this plugin allows you to do this:
https://wordpress.org/plugins/custom-css-js/
There are other plugins like this as well.
Can you annotate a screenshot of your form with what you want changing, and I can craft something for you? You cannot upload images here, so use a file sharing site and send me a link to the image.
If you are able to share the CSS you wrote, that might help also. It may be that the CSS lacked sufficient specificity to work.