Remove redundant charcoal color from our text

I've noticed that we're using two nearly identical charcoal colors for text. The body color is #444444 and some of classes overwrite it to #414141 for headings or bold text.

#### Steps to reproduce the issue

1. Go to Jetpack WP-Admin interface
2. The example is from At a Glance, but is prevalent elsewhere

#### What I expected
A single charcoal color across the interface

#### What happened instead
Two nearly identical colors being used interchangibly

#### Suggestion
Using the body color for text everywhere makes sense to me, but either charcoal works.

**Screenshots**


Start in the Jetpack WP-Admin interface by searching for the two colors, #444444 and #414141, across the relevant styles. The work is done when text uses one charcoal color consistently throughout the interface, including the At a Glance example.

Written by the indexing model from the issue text.