How do I modify the CSS in webform?
You'll find the .css files in your theme directory, usually sites/all/themes/THEME_NAME/css.

To see easily in which line of which css file is the rule you'd like to change, you can go to admin/config/development/performance and un-tick Aggregate and compress css files checkbox. 

Then with Firebug or similar development tool you'll be able to explore the css.

Check also if there's a sass or less folder in the same directory. In that case, you would have to edit the relevant scss/less files instead of the css ones, and make sure your editor is well configured to compile them to css. 

You can explore scss files with the help of FireCompass addd-on and less ones with FireLESS.

EDIT

As underlined by Beebee comment, if the css you'd like to override are into a module css file, such as webform's css, don't edit that file, but override it in your own theme's css file.