You can change font family and size like this:
app = dash.Dash(__name__)
app.layout = dash_table.DataTable(
id='table',
columns=[{"name": i, "id": i} for i in df.columns],
data=df.to_dict("rows"),
style_cell={'fontSize':20, 'font-family':'sans-serif'}
)
if __name__ == '__main__':
app.run_server(debug=True)