How would one override the hostname?

[ell1e]

How would one override the hostname in the title field, using the ~/config/fastfetch/config.jsonc? I've tried just replacing the entire field with a "custom" one, but then the "separator" below won't be the right size and the color coding is gone as well.

[CarterLi]

{
    "type": "separator",
    "times": 123 // your title length
}

[Dotoryman]

Keep the title module and customize its format, rather than replacing it with a custom module. That retains Fastfetch's title-specific user/@ coloring:

"modules": [
  {
    "type": "title",
    "format": "{user-name-colored}{at-symbol-colored}{#blue}MyHost"
  },
  { "type": "separator", "times": 12 }
]

Replace MyHost and set times to the displayed width of username@MyHost (12 is only an example). The earlier times suggestion is needed because the separator's automatic length is calculated from the real detected username and hostname, not the rendered title format. So there currently isn't an automatic separator length that follows an arbitrary replacement hostname; you have to set times when you change the displayed text.

{user-name-colored} and {at-symbol-colored} are built-in Title format fields, while {#blue} colors the literal replacement hostname. You can adjust that color to match your existing theme.

Sources: Title format fields, separator width calculation.