Very quickly

• Create the ~/.config/helix/themes directory.
• Create your custom theme; in my case, ayu_nb.toml.
• In it, inherit from the theme you’re currently using. And add the "ui.background" = {}

inherits = "ayu_dark"
"ui.background" = {}

• Edit your ~/.config/helix/config.toml to use your theme.

theme = "ayu_nb"

[...]

With these steps, you go from this

to this

note: ptyxis terminal with Ayu theme