In vim-airline palette the keyword for italic style is 'italic', whereas in tmux the accepted keyword is 'italics'. This will lead to an incorrect parse of the theme (colors) from :Tmuxline or call tmuxline#api#create_theme_from_airline(mode_palette), the color with an 'italic' attribute will be ignored, and a "bad style" error will be emitted by tmux if the corresponding .conf is sourced. I personally use vim-airline but i suppose it's a bug for all sources using 'italic' as keyword.