MacのAlacrittyのtmuxでneovim

Sometimes setting 'termguicolors' is not enough and one has to set the t_8f and t_8b options explicitly. Default values of these options are "^[[38;2;%lu;%lu;%lum" and "^[[48;2;%lu;%lu;%lum" respectively, but it is only set when $TERM is xterm. Some terminals accept the same sequences, but with all semicolons replaced by colons (this is actually more compatible, but less widely supported): 
         let &t_8f = "\<Esc>[38:2:%lu:%lu:%lum"
         let &t_8b = "\<Esc>[48:2:%lu:%lu:%lum"

（出典：VIM REFERENCE MANUAL）

init.vimに以下を設定します。

set termguicolors
let &t_8f = "\<Esc>[38:2:%lu:%lu:%lum"
let &t_8b = "\<Esc>[48:2:%lu:%lu:%lum"

~/.tmux.confに以下を設定します。

set -g default-terminal "screen-256color"
set-option -ga terminal-overrides ",xterm-256color:Tc"

これで綺麗に表示されました。