It's not a Helix problem, If I recall correctly it's about "true color (24 bits)" in tmux, you should try this on your tmux config https://gist.github.com/andersevenrud/015e61af2fd264371032763d4ed965b6#tmux
Sorry, currently I use Zellij, but I encountered this problem with tmux + neovim
An application to create and publish notes? Something like StandardNotes or Notesnook?