code: def test_themes():
      chart = alt.Chart("foo.txt").mark_point()
  
      with alt.themes.enable("default"):
          assert chart.to_dict()["config"] == {
              "view": {"continuousWidth": 300, "continuousHeight": 300}
          }
  
      with alt.themes.enable("opaque"):
          assert chart.to_dict()["config"] == {
              "background": "white",
              "view": {"continuousWidth": 300, "continuousHeight": 300},
          }
  
      with alt.themes.enable("none"):
          assert "config" not in chart.to_dict()
  
imports: import io
  import json
  import operator
  import os
  import pathlib
  import tempfile
  import jsonschema
  import pytest
  import pandas
  import altair.vegalite.v5
label_desc: Use image node_id 28 for calling a global function with example usage: test_themes() without return types