dcc redesign: Slider handles should appear visually different after mouse out

[KoolADE85]

Currently, the slider CSS is identical for both focus and hover states. We should have a slightly different look and feel so that users can visually distinguish between these actions:

• keyboard focus tabbed onto the slider handle
• mouse hovered onto the slider handle
• mouse hovered onto slider handle followed by click to adjust slider (note that a click will focus the slider, same as a keyboard tab would)
• while is slider handle is focused, mouse OFF of the handle

Screenshots:

Unfocused, unhovered:

Unfocused, hovered (thumb is slightly larger):

Focused, unhovered is the same as above: