Why does my dash app justify = "center" not work?

I am very new to Dash apps and I am writing a simple outline of how my app should look like.

my_simple_app = dash.Dash(external_stylesheets=[dbc.themes.BOOTSTRAP])

my_simple_app.layout = html.Div(
    children=[
        # First row with title
        dbc.Row(html.H1("This part should center"), justify="center", align="center", className="h-50"),

        # Thematic break
        html.Hr(),

        # Second row
        dbc.Row(children=[
            # First column: dropdown
            dbc.Col(html.Span("It will be my dropdown"), width=3),
            # Second column: graph
            dbc.Col(html.Span("It will be my graph"), width=9)
        ])
    ])

if __name__ == "__main__":
    my_simple_app.run_server()

However I am not able to display the header in center

How can I get the header to display at the center of the page?