Highlighting the first item created in the list - SharePoint Online (365)

ErikJacko your screenshot showed formatting on one of the "child" rows in the groupby view. But you can highlight which is the parent in the groupby by using the following JSON view formatting:

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/row-formatting.schema.json",
  "groupProps": {
    "hideFooter": false,
    "headerFormatter": {
      "elmType": "div",
      "style": {
        "flex-wrap": "wrap",
        "display": "flex",
        "box-sizing": "border-box",
        "padding": "4px 8px 5px 4px",
        "border-radius": "6px",
        "align-items": "center",
        "white-space": "nowrap",
        "overflow": "hidden",
        "margin": "10px 4px 4px 1px",
        "border": "3px solid gold"
      },
      "children": [
        {
          "elmType": "div",
          "children": [
            {
              "elmType": "div",
              "style": {
                "display": "flex",
                "flex-direction": "row",
                "justify-content": "center"
              },
              "children": [
                {
                  "elmType": "div",
                  "txtContent": "=  @group.count",
                  "style": {
                    "padding": "1px 5px 0px 5px",
                    "font-weight": "500"
                  }
                }
              ]
            }
          ]
        },
        {
          "elmType": "div",
          "children": [
            {
              "elmType": "div",
              "style": {
                "display": "flex",
                "flex-direction": "row",
                "justify-content": "center"
              },
              "children": [
                {
                  "elmType": "div",
                  "txtContent": "='x ' +  @group.fieldData.displayValue +'(s)  ' ",
                  "style": {
                    "padding": " ",
                    "font-weight": "500"
                  }
                }
              ]
            }
          ]
        }
      ]
    }
  }
}

This will give you the following result:

Rob
Los Gallardos
Intranet, SharePoint and Power Platform Manager (and classic 1967 Morris Traveller driver)