More distinguished toggleable button state

Use darker colors for .btn.active (i.e. pressed) state when button is set toggleable via data-toggle="button...".
Also after button is pressed it becomes brighter on :hover, :focus, :sctive.

So when button is in the released state hovering over makes it darker as default, but after clicking on it the color becomes even darker and it easy to see that the button is pressed.
On the other side, hovering over the pressed button makes it brighter and clicking on it brightens button further.

This helps not only see the current button state easier but also shows which way the state will change.