Use foo-style instead of foo-{fg,bg,attr} in .tmux.conf

Quote from tmux 1.9a CHANGES file
http://sourceforge.net/p/tmux/tmux-code/ci/master/tree/CHANGES

  The various foo-{fg,bg,attr} commands have been deprecated and replaced
  with equivalent foo-style option instead.  Currently this is still
  backwards-compatible, but will be removed over time.