Theming

You should never be modifying the Gravity Forms plugin files, or editing the Gravity Forms stylesheet itself. That isn't how you implement customizations, be it code customizations or style customizations using CSS.

If you want to style a form you would add custom CSS to your themes stylesheet to target and style the form elements you want to style. This is the proper way to style the forms and you won't lose anything when updating because your CSS is part of your theme and not part of the Gravity Forms plugin files.

There is a tutorial on how to target and style form elements int he documentation here:

http://www.gravityhelp.com/documentation/page/CSS_Targeting_Samples

This discusses how to write custom CSS to target and style any element in a form.

For code customizations you would implement customizations using hooks and filters by placing custom code in your themes functions.php file. Our developer documentation can be found here:

http://www.gravityhelp.com/documentation/page/Developer_Docs

So customizations (code or design customizations) should never be implemented by editing the Gravity Forms plugin files. They should always be done using the methods above.