Twenty Fifteen: improve playlist styles

[celloexpressions]

Playlist styles currently look kind of broken. I think more vertical padding between items in the tracklist, fixing the lines between tracks, and either colorscheme integration or an otherwise customized current-time-indicator (the default wp blue) will go a long way.

[celloexpressions]

Current playlist styles in Twenty Fifteen.

[iamtakashi]

I can make the style vertical spacing and borders better but I prefer leaving the indicator as default since there is no connection to any other custom color.

[iamtakashi]

Style tweak for playlist style.

[iamtakashi]

After:

[celloexpressions]

Looks much better! Since there isn't really an "accent" color in this theme, the default blue is fine for the current-time indicator.

[helen]

If it's of interest, I did some tweaking here in a child theme to make the player feel more inline: ​http://www.adriansandi.com/audio/

[iandstewart]

In 30213:

Twenty Fifteen: nicer styles for playlists.

Props iamtakashi, fixes #30163