TITLE: tmux no current window

set pane-active-border-style 'bg=default, fg=red'

set -g pane-active-border-style 'bg=default, fg=red'
     |
     x -> no current session

Each session option also has a global value that each session inherits by default. Use -g to change the global default. When setting a session option in your ~/.tmux.conf file you always want -g because there is no current session. Example: