terminalwriter: improve PYTEST_THEME, PYTEST_THEME_MODE usage errors

Author: bluetech

src/_pytest/_io/terminalwriter.py

@@ -235,13 +235,13 @@ def _get_pygments_formatter(self) -> Optional["Formatter"]:
 
         except pygments.util.ClassNotFound as e:
             raise UsageError(
-                f"PYTEST_THEME environment variable had an invalid value: '{theme}'. "
-                "Only valid pygment styles are allowed."
+                f"PYTEST_THEME environment variable has an invalid value: '{theme}'. "
+                "Hint: See available pygments styles with `pygmentize -L styles`."
             ) from e
         except pygments.util.OptionError as e:
             raise UsageError(
-                f"PYTEST_THEME_MODE environment variable had an invalid value: '{theme_mode}'. "
-                "The only allowed values are 'dark' and 'light'."
+                f"PYTEST_THEME_MODE environment variable has an invalid value: '{theme_mode}'. "
+                "The allowed values are 'dark' (default) and 'light'."
             ) from e
 
     def _highlight(

testing/test_terminal.py

@@ -2609,8 +2609,8 @@ def test_foo():
         monkeypatch.setenv("PYTEST_THEME", "invalid")
         result = pytester.runpytest_subprocess("--color=yes")
         result.stderr.fnmatch_lines(
-            "ERROR: PYTEST_THEME environment variable had an invalid value: 'invalid'. "
-            "Only valid pygment styles are allowed."
+            "ERROR: PYTEST_THEME environment variable has an invalid value: 'invalid'. "
+            "Hint: See available pygments styles with `pygmentize -L styles`."
         )
 
     def test_code_highlight_invalid_theme_mode(
@@ -2625,8 +2625,8 @@ def test_foo():
         monkeypatch.setenv("PYTEST_THEME_MODE", "invalid")
         result = pytester.runpytest_subprocess("--color=yes")
         result.stderr.fnmatch_lines(
-            "ERROR: PYTEST_THEME_MODE environment variable had an invalid value: 'invalid'. "
-            "The only allowed values are 'dark' and 'light'."
+            "ERROR: PYTEST_THEME_MODE environment variable has an invalid value: 'invalid'. "
+            "The allowed values are 'dark' (default) and 'light'."
         )