Normalize native select option theming (#1178)

Co-authored-by: ghreprimand <203024559+ghreprimand@users.noreply.github.com>

tests/test_select_dropdown_theme_css.py

@@ -0,0 +1,33 @@
+from pathlib import Path
+
+
+STYLE_CSS = Path(__file__).resolve().parents[1] / "static" / "style.css"
+
+
+def _style_text() -> str:
+    return STYLE_CSS.read_text(encoding="utf-8")
+
+
+def test_native_select_options_use_theme_tokens():
+    css = _style_text()
+
+    assert "--select-option-bg:" in css
+    assert "--select-option-fg:" in css
+    assert "--select-option-active-bg:" in css
+    assert "select option,\n    select optgroup" in css
+    assert "background-color: var(--select-option-bg);" in css
+    assert "color: var(--select-option-fg);" in css
+    assert "select option:checked" in css
+    assert "background-color: var(--select-option-active-bg);" in css
+
+
+def test_light_theme_keeps_native_selects_light():
+    css = _style_text()
+
+    light_theme_start = css.index(":root.light {")
+    light_theme_end = css.index("}", light_theme_start)
+    light_theme_block = css[light_theme_start:light_theme_end]
+
+    assert "--select-bg: #eaeaea;" in light_theme_block
+    assert "--select-option-bg: var(--panel);" in light_theme_block
+    assert ":root.light select { color-scheme: light; }" in css