You need to add a style parameter to apply styling and this can be conditional, as you see below. In style or class I have made certain changes which should work for you

    {
      "$schema": "https://developer.microsoft.com/json-schemas/sp/column-formatting.schema.json",
      "debugMode": true,
      "elmType": "div",
      "attributes": {
        "class": "=if(@currentField,'sp-field-severity--good','')"
      },
      "children": [
        {
          "elmType": "span",
          "style": {
            "display": "inline-block",
            "padding": "0 4px",
            "color": "=if([@currentField] == '', 'red', 'black')"
          }
        },
        {
          "elmType": "span",
          "txtContent": "@currentField"
        }
      ]
    }