I’m working on customizing the UI of Blender (version 4.2.0) and need to modify the default collapse arrow icon in a custom panel. By default, Blender uses its own collapse arrow. I want to replace this default collapse arrow with a custom icon and have it display TRIA_RIGHT when the panel is collapsed and TRIA_DOWN when it is expanded.

So the final panel should look someting like this
enter image description here

import bpy
from bpy.props import BoolProperty

class LayoutDemoPanel(bpy.types.Panel):
    bl_label = "Layout Panel Demo"
    bl_idname = "SCENE_PT_layout_panel"
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "scene"
    bl_options = {'HIDE_HEADER'}

    def draw_header(self, context):
        self.layout.label(text="", icon="TRIA_RIGHT")


    def draw(self, context):
        layout = self.layout
        scene = context.scene

        layout.label(text="Basic Panels")

        header, panel = layout.panel("my_panel_id", default_closed=True)
        header.label(text="Hello World")
        print(header.icon)
        if panel:
            panel.label(text="Success")

        header, panel = layout.panel_prop(scene, "show_demo_panel")
        
        # header.icon()
        if panel:
            header.label(text="My Panel", icon="TRIA_DOWN")
            panel.prop(scene, "frame_start")
            panel.prop(scene, "frame_end")
        else: header.label(text="My Panel", icon="TRIA_RIGHT")

        layout.label(text="Customized headers")

        # Add a checkbox to the Panel header. text must be None for this panel
        header, panel = layout.panel("my_panel_id-2", default_closed=True)
        header.prop(scene, "use_checkmark", text="") # text must be empty for the checkbox
        header.label(text="Checkmark at beginning")
        if panel:
            panel.label(text="Success")

        header, panel = layout.panel("my_panel_id-3", default_closed=True)
        header.label(text="Buttons at the end")
        header.operator("mesh.primitive_cube_add", text="", icon='EXPORT')
        header.operator("mesh.primitive_cube_add", text="", icon='X')
        if panel:
            panel.label(text="Success")

        header, panel = layout.panel("my_panel_id-4", default_closed=True)
        header.prop(scene, "use_checkmark2", text="")
        header.label(text="Both")
        header.operator("mesh.primitive_cube_add", text="", icon='EXPORT')
        header.operator("mesh.primitive_cube_add", text="", icon='X')
        if panel:
            panel.label(text="Success")

bpy.utils.register_class(LayoutDemoPanel)
bpy.types.Scene.show_demo_panel = BoolProperty(default=False)
bpy.types.Scene.use_checkmark = BoolProperty(default=False)
bpy.types.Scene.use_checkmark2 = BoolProperty(default=False)