Hover style for vote arrows

[gregmac]

Minor tweak: make the vote arrows change color on hover, since they're a small target.

This is very subtle, but changes the color of the arrow image to be #888888 which is same as the hover color of the "clickable indent" bar:

.votearrow:hover { filter: brightness(89%); }

I'd contribute as a PR but I'm not sure where to place it, or if this can just be a default vs optional feature.