This introduces --rb-p-draft-label-fg, which is a color used for draft

labels in conversations, helping visually distinguish published replies

from draft replies.
The color chosen, green-600, looks very close to the original color on

light mode and looks bright enough to contrast well with the background

in dark mode.
The Review Dialog will soon need this, so I'm splitting this work out

separatly from that change.

Tested in light and dark modes.