Reduce the default styling

[sergeyepimakhov]

The default styling in style.css that is used in the thebelab.js seems too customized. I guess we should keep the styling minimalistic. After importing thebelab into a web page some styling elements sometimes are very difficult to figure out and then to re-customize. This was the case, for example, for the "green border" of the active cell. Such things like the auto height of the CodeMirror area and disabling of prompts, in any case, must be kept, the rest is not so necessary to use. Let the user customize his own page further if desired. You can check the pull request on my fork for more details https://github.com/sergeyepimakhov/thebelab/pull/1/files