How do I change the style of the inner part enclosed by the lines(polar) chart created with highcharts?

Just use area series type instead of line:

  chart: {
    polar: true
  },
  series: [{
    type: 'area',
    data: [43934, 52503, 57177, 69658, 97031, 119931, 137133, 154175]
  }]

Live demo: http://jsfiddle.net/BlackLabel/67oxc5pt/

Docs: https://www.highcharts.com/docs/chart-concepts/series

Radar charts in Python with Plotly

For a filled line in a Radar Chart, update the figure created with px.line_polar with fig.update_traces

import plotly.express as px
import pandas as pd
df = pd.DataFrame(dict(
    r=[5, 4, 3, 1, 3],
    theta=['Social','Tech','Environmental',
           'Economical', 'Cultural']))
fig = px.line_polar(df, r='r', theta='theta', line_close=True)
fig.update_traces(fill='toself')
fig.show()

Output image