Update style.css

Updated style.css, fix for Firefox/Chrome users where text boxes can't be selected/focused using the mouse. This occurs when the captcha is used within bureacracy.

Fix provided by lupo49 

https://github.com/splitbrain/dokuwiki-plugin-captcha/issues/3
Merge pull request #26 from MartinMcGrath/patch-1

Update style.css (fix for issue #3)