I've used the following command to change the color of the status bar at the bottom of the screen:
set -g status-bg colour244
But I don't know how to change the color of the lines that divide the panes; currently, they're a mix of the original green and gray (color244).
man tmux gives me a lot of info about the status line but this seems to refer to the status bar itself, not the dividing lines.
I suspect I'm just missing some terminology here.