Background of meters feels a bit too dark now

Created by: pdubroy

Context: https://sourcegraph.slack.com/archives/C01TFNBHYCV/p1622223695112300

Change to the following:

Light theme:

• gray-04 by default (same as before the refresh)

• gray-05 for selected, active, hover states

  

Dark theme:

• gray-07 by default; unchanged in selected, active, hover states