Tests for background lightness checking.

src/napari_matplotlib/tests/test_theme.py

@@ -1,3 +1,4 @@
+import napari
 import pytest
 
 from napari_matplotlib.base import NapariMPLWidget
@@ -18,3 +19,33 @@ def test_theme_mpl_toolbar_icons(
     assert (
         path_to_icons.stem == expected_icons
     ), "The theme is selecting unexpected icons."
+
+
+def _mock_up_theme() -> None:
+    """Mock up a new color theme based on dark mode but with a tasteful blue background.
+
+    Based on:
+    https://napari.org/stable/gallery/new_theme.html
+    """
+    blue_theme = napari.utils.theme.get_theme("dark", False)
+    blue_theme.name = "blue"
+    blue_theme.background = "#4169e1"  # my favourite shade of blue
+    napari.utils.theme.register_theme("blue", blue_theme)
+
+
+def test_theme_background_check(make_napari_viewer):
+    """
+    Check that the hue saturation lightness can distinguish dark and light backgrounds.
+    """
+    viewer = make_napari_viewer()
+    widget = NapariMPLWidget(viewer)
+
+    viewer.theme = "dark"
+    assert widget._theme_has_light_bg() is False
+
+    viewer.theme = "light"
+    assert widget._theme_has_light_bg() is True
+
+    _mock_up_theme()
+    viewer.theme = "blue"
+    assert widget._theme_has_light_bg() is True