How to fill the white space

I want to fiil the yellow space on the same color to the background color(Gray) which is “#e5e8e6”.
Right now, the background color that I set as “#e5e8e6” does not reflect on the below white space as shown below.

The python code which generates the above picture is shown below.

import pandas as pd

import plotly.graph_objs as go
import dash
import dash_html_components as html
import dash_core_components as dcc

from dash.dependencies import Input, Output

app = dash.Dash(__name__)
# Get the dummy dataset
filepath = "df_visitor.csv"
df = pd.read_csv(filepath, parse_dates=["timestamp"]).set_index("timestamp")

x_date = df.index.tolist()
y1 = df["セッション数"]
y2 = df["新規"]
y3 = df["リピーター"]

labels = ["Organic Search", "Referral", "Display", "Direct", "Email", "Other Advertising"]
values = [17, 13, 20, 30, 10, 10]

# Define the interface
app.layout = html.Div([
    html.Div([html.H1("Dashboard for Google Analytics")],
             style={'padding': '20px', 'backgroundColor': '#2a3f5f',
                    "color": "white", "font-family": "Helvetica",
                    "font-weight": "bolder"}),

    dcc.Tabs(
        id="tabs-with-classes",
        value='tab-2',
        parent_className='custom-tabs',
        className='custom-tabs-container',
        style={"display": "flex", "font-size": "25px", "text-align": "center"},
        children=[
            dcc.Tab(
                label='訪問者数',
                value='tab-1',
                className='custom-tab',
                style={"flex": "1", "margin": "10px", "border": "solid 1px gray"},
                selected_className='custom-tab--selected'
            ),
            dcc.Tab(
                label='ユーザー数',
                value='tab-2',
                className='custom-tab',
                style={"flex": "1", "margin": "10px", "border": "solid 1px gray"},
                selected_className='custom-tab--selected'
            ),
            dcc.Tab(
                label='流入元',
                value='tab-3', className='custom-tab',
                selected_className='custom-tab--selected',
                style={"flex": "1", "margin": "10px", "border": "solid 1px gray"},
            ),

        ]),
    html.Div(id='tabs-content-classes')
], style={"background-color": "#e5e8e6"})


@app.callback(Output('tabs-content-classes', 'children'),
              [Input('tabs-with-classes', 'value')])
def render_content(tab):
    if tab == 'tab-1':
        return html.Div([
            html.Div([
                html.Div([
                    html.P("合計訪問者数"),
                    html.P(str(y1[-1]))],
                    className="session",
                    style={"flex": "1", "border": "solid 1px gray", "margin": "50px", "color": "#2a3f5f"}),

                html.Div([
                    html.P("新規訪問数"),
                    html.P(str(y2[-1]))],
                    className="new",
                    style={"flex": "1", "border": "solid 1px gray", "margin": "50px", "color": "#2a3f5f"}),

                html.Div([
                    html.P("リピート訪問数"),
                    html.P(str(142 - 21))],
                    className="repeat",
                    style={"flex": "1", "border": "solid 1px gray", "margin": "50px", "color": "#2a3f5f"})],
                className="three_indicators",
                style={"display": "flex", "font-size": "25px", "text-align": "center"}),

            html.Div([
                html.Div([
                    dcc.Graph(
                        id="visitor",
                        figure={
                            "data": [
                                go.Bar(
                                    x=x_date,
                                    y=y2,
                                    name="新規"
                                ),
                                go.Bar(
                                    x=x_date,
                                    y=y1 - y2,
                                    name="リピーター"
                                )],
                            'layout': go.Layout(
                                title="訪問者数(セッション数)の推移",
                                barmode="stack",
                                font=dict(size=20)
                            )
                        }
                    )
                ], style={"flex": "1", "border": "solid 1px gray", "margin": "25px"}),

                html.Div([

                    dcc.Graph(
                        figure={
                            "data": [
                                go.Pie(labels=labels, values=values,
                                       hoverinfo='label+percent', textinfo='value',
                                       textfont=dict(size=30),
                                       marker=dict(line=dict(color='#000000', width=5)))],
                            'layout': go.Layout(
                                title="流入元",
                                barmode="stack",
                                font=dict(size=20)
                            )
                        }
                    )

                ], style={"flex": "1", "border": "solid 1px gray", "margin": "25px"})

            ], style={"display": "flex", "font-size": "25px", "text-align": "center", "height": "100%"}),

            html.Div([
                html.Div([
                    dcc.Graph(
                        figure={
                            "data": [
                                go.Bar(
                                    x=x_date,
                                    y=y2,
                                    name="新規"
                                ),
                                go.Bar(
                                    x=x_date,
                                    y=y1 - y2,
                                    name="リピーター"
                                )],
                            'layout': go.Layout(
                                title="訪問者数(セッション数)の推移",
                                barmode="stack",
                                font=dict(size=20)
                            )
                        }
                    )
                ], style={"flex": "1", "border": "solid 1px gray", "margin": "25px"}),

                html.Div([

                    dcc.Graph(
                        figure={
                            "data": [
                                go.Pie(labels=labels, values=values,
                                       hoverinfo='label+percent', textinfo='value',
                                       textfont=dict(size=30),
                                       marker=dict(line=dict(color='#000000', width=5)))],
                            'layout': go.Layout(
                                title="流入元",
                                barmode="stack",
                                font=dict(size=20)
                            )
                        }
                    )

                ], style={"flex": "1", "border": "solid 1px gray", "margin": "25px"})

            ], style={"display": "flex", "font-size": "25px", "text-align": "center", "height": "100%"})
        ])


    elif tab == 'tab-2':
        return html.Div([
            html.H3('Tab content 2'),
        ])

    elif tab == 'tab-3':
        return html.Div([
            html.H3('Tab content 3')
        ])


if __name__ == '__main__':
    app.run_server(debug=True, port=9000)

I think you need to set the default background color for the whole webpage, and not just your Div containers.

I do not know much about css so I cannot provide exampel code, sorry.

I think you need to do something like the app.head in this example:

and then put in the background color in the style argument there, see:
https://www.w3schools.com/cssref/pr_background-color.asp