diff --git a/mkdocs/config/base.py b/mkdocs/config/base.py
index bd875191..f4e8279e 100644
--- a/mkdocs/config/base.py
+++ b/mkdocs/config/base.py
@@ -4,13 +4,27 @@ import functools
 import logging
 import os
 import sys
+import warnings
 from collections import UserDict
 from contextlib import contextmanager
-from typing import IO, TYPE_CHECKING, Generic, Iterator, List, Sequence, Tuple, TypeVar, overload
+from typing import (
+    IO,
+    TYPE_CHECKING,
+    Any,
+    Generic,
+    Iterator,
+    List,
+    Mapping,
+    Sequence,
+    Tuple,
+    TypeVar,
+    overload,
+)
 
 from yaml import YAMLError
 
 from mkdocs import exceptions, utils
+from mkdocs.utils import weak_property
 
 if TYPE_CHECKING:
     from mkdocs.config.defaults import MkDocsConfig
@@ -143,7 +157,7 @@ class Config(UserDict):
 
     def __init__(self, config_file_path: str | bytes | None = None):
         super().__init__()
-        self.user_configs: list[dict] = []
+        self.__user_configs: list[dict] = []
         self.set_defaults()
 
         self._schema_keys = {k for k, v in self._schema}
@@ -237,7 +251,7 @@ class Config(UserDict):
                 f"{type(patch)}"
             )
 
-        self.user_configs.append(patch)
+        self.__user_configs.append(patch)
         self.update(patch)
 
     def load_file(self, config_file: IO) -> None:
@@ -250,6 +264,13 @@ class Config(UserDict):
                 f"MkDocs encountered an error parsing the configuration file: {e}"
             )
 
+    @weak_property
+    def user_configs(self) -> Sequence[Mapping[str, Any]]:
+        warnings.warn(
+            "user_configs is never used in MkDocs and will be removed soon.", DeprecationWarning
+        )
+        return self.__user_configs
+
 
 @functools.lru_cache(maxsize=None)
 def get_schema(cls: type) -> PlainConfigSchema:
diff --git a/mkdocs/localization.py b/mkdocs/localization.py
index 73391489..59bb6ed3 100644
--- a/mkdocs/localization.py
+++ b/mkdocs/localization.py
@@ -33,7 +33,7 @@ class NoBabelExtension(InternationalizationExtension):  # pragma: no cover
         )
 
 
-def parse_locale(locale) -> Locale:
+def parse_locale(locale: str) -> Locale:
     try:
         return Locale.parse(locale, sep='_')
     except (ValueError, UnknownLocaleError, TypeError) as e:
diff --git a/mkdocs/tests/__init__.py b/mkdocs/tests/__init__.py
index 53d6d1a4..e0248786 100644
--- a/mkdocs/tests/__init__.py
+++ b/mkdocs/tests/__init__.py
@@ -1,4 +1,7 @@
 import logging
+import unittest.util
+
+unittest.util._MAX_LENGTH = 100000
 
 
 class DisallowLogsHandler(logging.Handler):
diff --git a/mkdocs/tests/config/config_options_tests.py b/mkdocs/tests/config/config_options_tests.py
index 80588c7d..c8dbff0b 100644
--- a/mkdocs/tests/config/config_options_tests.py
+++ b/mkdocs/tests/config/config_options_tests.py
@@ -1104,6 +1104,7 @@ class ThemeTest(TestCase):
 
         conf = self.get_config(Schema, {'option': config})
         self.assertEqual(conf.option.name, 'mkdocs')
+        self.assertEqual(conf.option.custom_dir, custom_dir)
         self.assertIn(custom_dir, conf.option.dirs)
         self.assertEqual(
             conf.option.static_templates,
@@ -1228,7 +1229,7 @@ class ThemeTest(TestCase):
             theme = c.Theme()
 
         conf = self.get_config(Schema, config)
-        self.assertEqual(conf.theme['locale'].language, 'fr')
+        self.assertEqual(conf.theme.locale.language, 'fr')
 
 
 class NavTest(TestCase):
diff --git a/mkdocs/theme.py b/mkdocs/theme.py
index a8e8ed13..c58a2f7b 100644
--- a/mkdocs/theme.py
+++ b/mkdocs/theme.py
@@ -2,7 +2,7 @@ from __future__ import annotations
 
 import logging
 import os
-from typing import Any
+from typing import Any, Collection, MutableMapping
 
 import jinja2
 
@@ -13,25 +13,32 @@ from mkdocs.utils import filters
 log = logging.getLogger(__name__)
 
 
-class Theme:
+class Theme(MutableMapping[str, Any]):
     """
     A Theme object.
 
-    Keywords:
-
+    Parameters:
         name: The name of the theme as defined by its entrypoint.
-
         custom_dir: User defined directory for custom templates.
-
         static_templates: A list of templates to render as static pages.
 
     All other keywords are passed as-is and made available as a key/value mapping.
-
     """
 
-    def __init__(self, name: str | None = None, **user_config) -> None:
+    def __init__(
+        self,
+        name: str | None = None,
+        *,
+        custom_dir: str | None = None,
+        static_templates: Collection[str] = (),
+        locale: str | None = None,
+        **user_config,
+    ) -> None:
         self.name = name
-        self._vars = {'name': name, 'locale': 'en'}
+        self._custom_dir = custom_dir
+        _vars: dict[str, Any] = {'name': name, 'locale': 'en'}
+        # _vars is soft-deprecated, intentionally hide it from mypy.
+        setattr(self, '_vars', _vars)
 
         # MkDocs provided static templates are always included
         package_dir = os.path.abspath(os.path.dirname(__file__))
@@ -41,8 +48,8 @@ class Theme:
         # Build self.dirs from various sources in order of precedence
         self.dirs = []
 
-        if 'custom_dir' in user_config:
-            self.dirs.append(user_config.pop('custom_dir'))
+        if custom_dir is not None:
+            self.dirs.append(custom_dir)
 
         if name:
             self._load_theme_config(name)
@@ -51,32 +58,54 @@ class Theme:
         self.dirs.append(mkdocs_templates)
 
         # Handle remaining user configs. Override theme configs (if set)
-        self.static_templates.update(user_config.pop('static_templates', []))
-        self._vars.update(user_config)
+        self.static_templates.update(static_templates)
+        _vars.update(user_config)
 
         # Validate locale and convert to Locale object
-        self._vars['locale'] = localization.parse_locale(self._vars['locale'])
+        _vars['locale'] = localization.parse_locale(
+            locale if locale is not None else _vars['locale']
+        )
+
+    name: str | None
+
+    @property
+    def locale(self) -> localization.Locale:
+        return self['locale']
+
+    @property
+    def custom_dir(self) -> str | None:
+        return self._custom_dir
+
+    dirs: list[str]
+
+    static_templates: set[str]
 
     def __repr__(self) -> str:
-        return "{}(name='{}', dirs={}, static_templates={}, {})".format(
+        return "{}(name={!r}, dirs={!r}, static_templates={!r}, {})".format(
             self.__class__.__name__,
             self.name,
             self.dirs,
-            list(self.static_templates),
-            ', '.join(f'{k}={v!r}' for k, v in self._vars.items()),
+            self.static_templates,
+            ', '.join(f'{k}={v!r}' for k, v in self.items()),
         )
 
     def __getitem__(self, key: str) -> Any:
-        return self._vars[key]
+        return self._vars[key]  # type: ignore[attr-defined]
 
-    def __setitem__(self, key, value):
-        self._vars[key] = value
+    def __setitem__(self, key: str, value):
+        self._vars[key] = value  # type: ignore[attr-defined]
 
-    def __contains__(self, item: str) -> bool:
-        return item in self._vars
+    def __delitem__(self, key: str):
+        del self._vars[key]  # type: ignore[attr-defined]
+
+    def __contains__(self, item: object) -> bool:
+        return item in self._vars  # type: ignore[attr-defined]
+
+    def __len__(self):
+        return len(self._vars)  # type: ignore[attr-defined]
 
     def __iter__(self):
-        return iter(self._vars)
+        return iter(self._vars)  # type: ignore[attr-defined]
 
     def _load_theme_config(self, name: str) -> None:
         """Recursively load theme and any parent themes."""
@@ -108,7 +137,7 @@ class Theme:
             self._load_theme_config(parent_theme)
 
         self.static_templates.update(theme_config.pop('static_templates', []))
-        self._vars.update(theme_config)
+        self._vars.update(theme_config)  # type: ignore[attr-defined]
 
     def get_env(self) -> jinja2.Environment:
         """Return a Jinja environment for the theme."""
@@ -117,5 +146,5 @@ class Theme:
         # No autoreload because editing a template in the middle of a build is not useful.
         env = jinja2.Environment(loader=loader, auto_reload=False)
         env.filters['url'] = filters.url_filter
-        localization.install_translations(env, self._vars['locale'], self.dirs)
+        localization.install_translations(env, self.locale, self.dirs)
         return env