Tweak default grey to be darker, possibly colder

[emilyhobson]

panel on the left and text headers on the table
Consider making higher constrast globally and possibly be more harmonious with grey used on map

[almccon]

Currently the panel on the left is defined here: https://github.com/cityenergyproject/seattle/blob/master/src/styles/main/_filter.scss#L134
@include linear-gradient(to right, #dedede, #d0d0d0);

The text seems to be #d1d1d0 from here: https://github.com/cityenergyproject/seattle/blob/master/src/styles/base/_variables.scss#L21

If we try to match the map, here are a few colors to try:
#a4afb0 is the dark gray of the coastline in the basemap.
#cdd2d4 is the blue-gray of the ocean on the map

Option 1 is #a4afb0

Option 2 is #cdd2d4

Option 3 is just darkening the existing gray, which I think works better:

Option 4 is the same as Option 3, but I'm darkening the pop-up, too. I think that makes the popup harder to see, however.

Going to implement Option 3 for now.