diff --git a/properdocs/config/config_options.py b/properdocs/config/config_options.py
index 952efe49..d16faea7 100644
--- a/properdocs/config/config_options.py
+++ b/properdocs/config/config_options.py
@@ -814,7 +814,10 @@ def run_validation(self, value: object) -> theme.Theme:
         if theme_config['name'] is not None and theme_config['name'] not in themes:
             message = f"Unrecognised theme name: '{theme_config['name']}'."
             if theme_config['name'] in ('mkdocs', 'readthedocs'):
-                message += f"\nAdditional dependency is needed:\n    pip install properdocs-theme-{theme_config['name']}"
+                message += (
+                    f"\nAn additional dependency is needed:"
+                    f"\n    pip install properdocs-theme-{theme_config['name']}"
+                )
             elif themes:
                 message += f" The available installed themes are: {', '.join(themes)}"
             else:
@@ -842,6 +845,21 @@ def run_validation(self, value: object) -> theme.Theme:
         return theme.Theme(**theme_config)
 
 
+class ProperDocsTheme(Theme):
+    def run_validation(self, value: object) -> theme.Theme:
+        if value is None:
+            value = 'mkdocs'
+            if self.config_file_path:
+                config_file_name = repr(os.path.basename(self.config_file_path))
+            else:
+                config_file_name = "the configuration file"
+            log.warning(
+                f"Please select a theme explicitly in {config_file_name}."
+                " Defaulted to 'theme: mkdocs', but this may change in the future."
+            )
+        return super().run_validation(value)
+
+
 class Nav(OptionallyRequired):
     """
     Nav Config Option.
diff --git a/properdocs/config/defaults.py b/properdocs/config/defaults.py
index db1f6956..a257a165 100644
--- a/properdocs/config/defaults.py
+++ b/properdocs/config/defaults.py
@@ -71,7 +71,7 @@ class ProperDocsConfig(base.Config):
     site_author = c.Optional(c.Type(str))
     """The name of the author to add to the HTML meta tags."""
 
-    theme = c.Theme(default='mkdocs')
+    theme = c.ProperDocsTheme()
     """The ProperDocs theme for the documentation."""
 
     docs_dir = c.DocsDir(default='docs', exists=True)
diff --git a/properdocs/tests/base.py b/properdocs/tests/base.py
index 3e6fe14e..d1be0ccb 100644
--- a/properdocs/tests/base.py
+++ b/properdocs/tests/base.py
@@ -26,8 +26,8 @@ def get_markdown_toc(markdown_source):
 def load_config(config_file_path: str | None = None, **cfg) -> ProperDocsConfig:
     """Helper to build a simple config for testing."""
     path_base = os.path.join(os.path.abspath(os.path.dirname(__file__)), 'integration', 'minimal')
-    if 'site_name' not in cfg:
-        cfg['site_name'] = 'Example'
+    cfg.setdefault('site_name', 'Example')
+    cfg.setdefault('theme', 'mkdocs')
     if 'docs_dir' not in cfg:
         # Point to an actual dir to avoid a 'does not exist' error on validation.
         cfg['docs_dir'] = os.path.join(path_base, 'docs')
diff --git a/properdocs/tests/build_tests.py b/properdocs/tests/build_tests.py
index 07cf45e4..9058b951 100644
--- a/properdocs/tests/build_tests.py
+++ b/properdocs/tests/build_tests.py
@@ -909,6 +909,7 @@ def test_markdown_extension_with_relative(self, config_dir):
             with self.subTest(base_path=base_path):
                 cfg = f'''
                     site_name: test
+                    theme: mkdocs
                     use_directory_urls: false
                     markdown_extensions:
                       - properdocs.tests.build_tests:
diff --git a/properdocs/tests/config/base_tests.py b/properdocs/tests/config/base_tests.py
index 42c3eea7..2463daaf 100644
--- a/properdocs/tests/config/base_tests.py
+++ b/properdocs/tests/config/base_tests.py
@@ -46,7 +46,7 @@ def test_load_from_file(self, temp_dir):
         Allows users to specify a config other than the default `properdocs.yml`.
         """
         with open(os.path.join(temp_dir, 'properdocs.yml'), 'w') as config_file:
-            config_file.write("site_name: ProperDocs Test\n")
+            config_file.write("site_name: ProperDocs Test\ntheme: mkdocs\n")
         os.mkdir(os.path.join(temp_dir, 'docs'))
 
         cfg = base.load_config(config_file=config_file.name)
@@ -57,7 +57,7 @@ def test_load_from_file(self, temp_dir):
     def test_load_default_file(self, temp_dir):
         """Test that `properdocs.yml` will be loaded when '--config' is not set."""
         with open(os.path.join(temp_dir, 'properdocs.yml'), 'w') as config_file:
-            config_file.write("site_name: ProperDocs Test\n")
+            config_file.write("site_name: ProperDocs Test\ntheme: mkdocs\n")
         os.mkdir(os.path.join(temp_dir, 'docs'))
         with change_dir(temp_dir):
             cfg = base.load_config(config_file=None)
@@ -68,7 +68,7 @@ def test_load_default_file(self, temp_dir):
     def test_load_default_file_with_yaml(self, temp_dir):
         """Test that `properdocs.yml` will be loaded when '--config' is not set."""
         with open(os.path.join(temp_dir, 'properdocs.yaml'), 'w') as config_file:
-            config_file.write("site_name: ProperDocs Test\n")
+            config_file.write("site_name: ProperDocs Test\ntheme: mkdocs\n")
         os.mkdir(os.path.join(temp_dir, 'docs'))
         with change_dir(temp_dir):
             cfg = base.load_config(config_file=None)
@@ -79,9 +79,9 @@ def test_load_default_file_with_yaml(self, temp_dir):
     def test_load_default_file_prefer_yml(self, temp_dir):
         """Test that `properdocs.yml` will be loaded when '--config' is not set."""
         with open(os.path.join(temp_dir, 'properdocs.yml'), 'w') as config_file1:
-            config_file1.write("site_name: ProperDocs Test1\n")
+            config_file1.write("site_name: ProperDocs Test1\ntheme: mkdocs\n")
         with open(os.path.join(temp_dir, 'properdocs.yaml'), 'w') as config_file2:
-            config_file2.write("site_name: ProperDocs Test2\n")
+            config_file2.write("site_name: ProperDocs Test2\ntheme: mkdocs\n")
 
         os.mkdir(os.path.join(temp_dir, 'docs'))
         with change_dir(temp_dir):
@@ -104,7 +104,15 @@ def test_load_from_open_file(self, temp_path):
         config_file.flush()
         os.mkdir(os.path.join(temp_path, 'docs'))
 
-        cfg = base.load_config(config_file=config_file)
+        with self.assertLogs('properdocs') as cm:
+            cfg = base.load_config(config_file=config_file)
+        self.assertEqual(
+            cm.output,
+            [
+                "WARNING:properdocs.config.config_options:Please select a theme explicitly in 'properdocs.yml'. Defaulted to 'theme: mkdocs', but this may change in the future."
+            ],
+        )
+
         self.assertTrue(isinstance(cfg, defaults.ProperDocsConfig))
         self.assertEqual(cfg.site_name, 'ProperDocs Test')
         # load_config will always close the file
@@ -117,7 +125,7 @@ def test_load_from_closed_file(self, temp_dir):
         Ensure `load_config` reloads the closed file.
         """
         with open(os.path.join(temp_dir, 'properdocs.yml'), 'w') as config_file:
-            config_file.write("site_name: ProperDocs Test\n")
+            config_file.write("site_name: ProperDocs Test\ntheme: mkdocs\n")
         os.mkdir(os.path.join(temp_dir, 'docs'))
 
         cfg = base.load_config(config_file=config_file)
@@ -248,7 +256,7 @@ def test_load_from_file_with_relative_paths(self, config_dir):
         """
         config_fname = os.path.join(config_dir, 'properdocs.yml')
         with open(config_fname, 'w') as config_file:
-            config_file.write("docs_dir: src\nsite_name: ProperDocs Test\n")
+            config_file.write("docs_dir: src\nsite_name: ProperDocs Test\ntheme: mkdocs\n")
         docs_dir = os.path.join(config_dir, 'src')
         os.mkdir(docs_dir)
 
diff --git a/properdocs/tests/config/config_tests.py b/properdocs/tests/config/config_tests.py
index b0358b17..4706cf82 100644
--- a/properdocs/tests/config/config_tests.py
+++ b/properdocs/tests/config/config_tests.py
@@ -29,13 +29,27 @@ def test_missing_config_file(self):
 
     def test_missing_site_name(self):
         conf = defaults.ProperDocsConfig()
-        conf.load_dict({})
+        conf.load_dict({'theme': 'mkdocs'})
         errors, warnings = conf.validate()
         self.assertEqual(
             errors, [('site_name', ValidationError("Required configuration not provided."))]
         )
         self.assertEqual(warnings, [])
 
+    def test_missing_theme(self):
+        conf = defaults.ProperDocsConfig(config_file_path='a.yml')
+        conf.load_dict({'site_name': 'Example'})
+        with self.assertLogs('properdocs') as cm:
+            errors, warnings = conf.validate()
+        self.assertEqual(errors, [])
+        self.assertEqual(warnings, [])
+        self.assertEqual(
+            cm.output,
+            [
+                "WARNING:properdocs.config.config_options:Please select a theme explicitly in 'a.yml'. Defaulted to 'theme: mkdocs', but this may change in the future."
+            ],
+        )
+
     def test_nonexistant_config(self):
         with self.assertRaises(ConfigurationError):
             config.load_config(config_file='/path/that/is/not/real')
@@ -69,6 +83,7 @@ def test_config_option(self, temp_path):
         file_contents = dedent(
             """
             site_name: Example
+            theme: mkdocs
             nav:
             - 'Introduction': 'index.md'
             """
@@ -235,7 +250,7 @@ def test_empty_nav(self):
         conf = defaults.ProperDocsConfig(
             config_file_path=os.path.join(os.path.abspath('.'), 'properdocs.yml')
         )
-        conf.load_dict({'site_name': 'Example'})
+        conf.load_dict({'site_name': 'Example', 'theme': 'mkdocs'})
         conf.validate()
         self.assertEqual(conf['nav'], None)
 
@@ -244,6 +259,7 @@ def test_error_on_pages(self):
         conf.load_dict(
             {
                 'site_name': 'Example',
+                'theme': 'mkdocs',
                 'pages': ['index.md', 'about.md'],
             }
         )
diff --git a/properdocs/tests/structure/page_tests.py b/properdocs/tests/structure/page_tests.py
index 6c58385f..4d6810e9 100644
--- a/properdocs/tests/structure/page_tests.py
+++ b/properdocs/tests/structure/page_tests.py
@@ -20,6 +20,7 @@
 
 def load_config(**cfg) -> ProperDocsConfig:
     cfg.setdefault('site_name', 'Example')
+    cfg.setdefault('theme', 'mkdocs')
     cfg.setdefault(
         'docs_dir',
         os.path.join(