With this change, all CSS border declarations that specify some shade of
gray are modified to use the one unified --border-color declaration in
app-theme.html.
Further followup with UX is required to ensure that the shade of gray
chosen is the correct one -- for now, the most-used one is used.
Change-Id: I2134ebf4a459ba040e01856c11389780307d8648
c828098853