Fixes the hover state on buttons for the Bootstrap 2 theme.

Hovering on the "theme buttons" produces an imperceptible difference over the
normal state (the border darkens slightly). This makes it difficult to see
if you are hovering over a button or not. The fix just changes the gradient
to a darker variation when hovering and/or active.

Issues #9879 and #9877

(there's also an accidental white space change. sorry!)