tmux: move from *-bg,fg to *-style options

As noted in tmux 2.9 release notes in CHANGES file:

> The individual -fg, -bg and -attr options have been removed;
> they were superseded by -style options in tmux 1.9.

See also:

https://github.com/tmux/tmux/wiki/FAQ#how-do-i-translate--fg--bg-and--attr-options-into--style-options

Author: marbu

.tmux.conf

@@ -19,8 +19,7 @@ set -g status-bg red
 set -g status-fg white
 
 # status line: highlight active window
-set-window-option -g window-status-current-bg white
-set-window-option -g window-status-current-fg red
+set-window-option -g window-status-current-style fg=red,bg=white
 
 # Rather than constraining window size to the maximum size of
 # any client connected to the *session*, constrain window size