Change SharePoint library column JSON to highlight dates in the last 24 hrs to a different colour

Try this:

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "div",
  "style": {
    "box-sizing": "border-box",
    "padding": "0 2px",
    "overflow": "hidden",
    "text-overflow": "ellipsis",
    "background-color": {
      "operator": "?",
      "operands": [
        {
          "operator": "<=",
          "operands": [
            {
              "operator": "-",
              "operands": [
                {
                  "operator": "Number",
                  "operands": [
                    {
                      "operator": "Date()"
                    }
                  ]
                },
                {
                  "operator": "Number",
                  "operands": [
                    {
                      "operator": "Date()",
                      "operands": [
                        "[$Last_x0020_Posted_x0020_Date]"
                      ]
                    }
                  ]
                }
              ]
            },
            86400000
          ]
        },
        "darkgreen",   // Highlight files posted in the last 24 hours
        {
          "operator": "?",
          "operands": [
            {
              "operator": "!=",
              "operands": [
                "[$Last_x0020_Posted_x0020_Date]",
                null
              ]
            },
            "lightgreen", // Highlight files with a valid date
            "transparent" // Default background for files with no date
          ]
        }
      ]
    },
    "color": "white"
  },
  "children": [
    {
      "elmType": "span",
      "style": {
        "overflow": "hidden",
        "text-overflow": "ellipsis",
        "padding": "0 3px"
      },
      "txtContent": "[$Last_x0020_Posted_x0020_Date.displayValue]"
    }
  ]
}