correct background-color of message--editing element

[mastermel]

Fixes #164

Before: https://cl.ly/3a3f0O0z2A1F After: https://cl.ly/3v03280i3M14

This updates the background color and border of the div responsible for editing an existing message.

Test Plan:

• Send a message to a Channel or Direct Message
• Edit your own message (press the up arrow, or use the hover menu on the message)
• Verify that the background color of the editing element is grey instead of yellow