[tmux] make the status easier to read
[tmux] change the colors of borders; add comments for OS X.

For some reasons, when using Terminal.app on OS X 10.9 checking
"Unicode East Asian Ambiguous characters are wide", the vertical
borders between splitted panes get invisible. When you switch between
panes, the borders can be observed for a moment. Apparently, to make
it visible all the time, unchecking the preference seems to be
required.