eliminate startup toolbar flicker for hidden mode

In hidden mode, the toolbar is given `hidden` class at startup, which introduces a 200ms delay in which the toolbar slowly fades in. This patch eliminates this issue. 

The fix involves merging the `hidden` class with the default styles, so the animation does not trigger at startup, and adding a `show` class for non-hidden mode.

The folowing files are changed:
* script.js : switching `hidden` and `show`
* style.css: see above