tmux: use foo-style to set status styles

tmux: use foo-style to set status styles

From tmux 1.9, (20 Feb 2014), foo-{bg,fg,attr} commands have been
deprecated and tmux recommended to use foo-style instead.

Those options have been removed from tmux 2.9

It's more than 5 years from the deprecation of foo-{bg,fg,attr}.
it should be safe to use the new option by now.

Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>