The view source view is a combination of code from your content, your theme (a variety of files from your theme, such as header.php, index.php, footer.php, etc), plugins, etc
style.css is your theme’s main css file
plugins may use their own
your theme may use additional css files
you may have read this already:
http://codex.wordpress.org/Editing_Files
in a firefox browser, you can view the css files with a tool such as firefox web developer add-on; and there might be similar tools for other browsers.
some of them let you ‘edit’ the css in real time to try changes or to troubleshoot your site’s css – however, these tools cannot save the edits to your theme’s stylesheets (css files).
Thank You, for both, Hein