Changes to v2.9
From the tmux changes from 2.8 to 2.9:
“The individual -fg, -bg and -attr options have been removed; they were superseded by -style options in tmux 1.9.”
So
set-window-option -g window-status-current-bg red
set-window-option -g window-status-current-fg black
becomes
set-window-option -g window-status-current-style bg=red,fg=black
and
set -g status-attr default
becomes
set -g status-style default