MDL-63042 sets bg on dasboard to light gray

MDL-63042 set the background color on #page-my-index to a light shade of gray ($gray-100: #f8f9fa).

This means if you set the $body-bg variable, then every page background will change apart from this one.

I'm not totally sure what the reasoning behind the color choice is (possibly to set off the blocks from the white background better but one option is to set a variable for this (name will probably depend on the exact reason for the choice):

$dashboard-bg: darken($body-bg, 10%) !default;

This will be the same color as now if the body-bg is white, and it should work for most other light shades as well. If someone wants to use a very dark background then they may need to override this variable.