What is the setting that is making this awful yellow?

[SephReed]

I've commented out every line in my theme. Absolutely none of them are doing this. How do I force this thing to not be bright yellow?

[UNovak]

I think yellow underline is a warning, could also be info or hint. Not sure.
If you have a custom theme this element can be modified by changing the warning properties under style in your theme file:

"themes": [
  {
    "theme": "YourTheme",
    "appearance": "light",
    "style": {
      // ...
      "warning.background": "#c18f41d3"
      //...
    }
  },
  // ...
]

If you have a theme from an extension you can override only specific properties for a theme in your zed settings.json file like this:

"experimental.theme_overrides": {
    // underline
    "error": "#b64848",
    // popup background
    "error.background": "#643838",
     // popup outline
    "error.border": "#837373",
    // text colour
    "editor.foreground": "#fff"
}

For more detail see the docs
or read the recommended blog