If you're using tmux, it's worth checking that 24 bit color is working correctly. For example, run a color test script like this one, or one of the others listed here. If you do not see smooth color gradients, see the discussion at tmux#696. The short version is you need something like this in your
set -ga terminal-overrides ",xterm-256color:Tc"
and you may then need to quit tmux completely for it to take effect.