When I set any theme with a dark background in vim, on opening any file in tmux, the background spreads only around the lines having text. (Please see the attached image)
my tmux.conf has set-option -g default-terminal "xterm-256color"
and my .cshrc file has setenv TERM xterm-256color
I think, this question is answered here.
The fix is to disable the Background Color Erase Property for the terminal:
:set t_ut=