Clean up system.module.css

Sub-issue of #1190252: [573] Use csslint as a weapon to beat the crappy CSS out of Drupal core →

Inline with the CSS cleanup efforts of the HTML5 initiative, using CSSLint at http://csslint.net provides a quick way to code-sniff our css and tweak styles.

1. Copy and paste the the stylesheet(s) below into the css lint tool at http://csslint.net and test.
2. Fix any warnings or errors the tool finds.
3. Patch Drupal 8 locally and make sure the css changes have not broken anything visually.
4. Create patch and upload for the testbot.

Files: modules/system/system.module.css (and system.module-rtl.css)