Update alertify.default.css

https://github.com/fabien-d/alertify.js/issues/187 : submit a base theme;

revised top-level .alertify rules and moved background rules to its children;

removed border styling except for <input> element; perhaps bump up the border width from 2px to 3px to really make it look like a text <input>

revised .alertify-buttons to use avoid using position styles;
added color palette for button-cancel and button-ok;

revised .alertify-title to override default margins inherited from <p> tags and bumped up its font-size slightly to attract more attention to the message;

added vendor prefixes for box-sizing; added IE fallback for backgrounds with opacity;

removed .alertify--alert .alertify-buttons:after rule because of .alertify-buttons revisions;