Updated CSS for buttons and inputs.

CSS now applies to all buttons and inputs, not just those found inside a
<form>. (This makes the CSS selector shorter, and also makes the CSS
apply to web apps -- where these inputs control javascript behavior, and
do not necessarily appear inside a <form>.)
Updated input field styles + button shadows.

* Textareas and contenteditable fields are now styled in the same way as other
  text fields.

* When clicking a button, the whole button is darkened (both text & background,
  not just the background).

* Disabled buttons are now shaded (unfortunately hovering is still indicated,
  it was simply to messy to remove from the CSS).