Reapply initial tmux scheme in modern config

Summary:
The tmux initial scheme is:

• main tmux should use dark gray bar
• other tmux should use light gray bar

Update the configuration to recent tmux expected version.

Test Plan: Tested on WindRiver

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2311