Style scrollbar slider
Style scrollbar slider

Rather than sinking time into styling the Webkit scrollbar right now,
I discussed this solution with Mac: style the trough of the text cards
scrollbar per the design specs, but style the slider to look more
unified with the Webkit scrollbar.

This is temporary; eventually they should both look like the design
specs.

[endlessm/eos-sdk#1661]

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib