theme: use $TMUX for _USE_XTERM_CURSOR_CODES detection
theme: use $TMUX for _USE_XTERM_CURSOR_CODES detection

tmux will use its Ss/Se terminfo extension (via terminal-overrides) to
translate this.

blueyed/oh-my-zsh,blueyed/oh-my-zsh,blueyed/oh-my-zsh