Custom Box Layout header and collapse

Did you try to just set the Style to the white?

Please check the color of box layout in the style section. 

Please try the below code

{
  a!horizontalLine(),
  a!boxLayout(
    label: "Remarks",
    labelSize: "SMALL",
    isCollapsible: true,
    style: "#ffffff",
    showBorder: false(),
    contents: a!richTextDisplayField(
      value: {
        a!richTextItem(
          text: "Remark",
          style: "STRONG"
        ),
        char(10),
        a!richTextItem(
          text: "Need to check customer claim history"
        )
      }
    )
  ),
  a!horizontalLine(),
  a!boxLayout(
    label: "Document",
    labelSize: "SMALL",
    isCollapsible: true,
    style: "#ffffff",
    showBorder: false()
    )
}