Missing theme config file raises an error.

Previously a warning was raised and the build proceeded.

docs/about/release-notes.md

@@ -229,7 +229,7 @@ authors should review how [search and themes] interact.
 [search config]: ../user-guide/configuration.md#search
 [search and themes]: ../user-guide/custom-themes.md#search_and_themes
 
-#### `theme_dir` Configuration Option fully Deprecated.
+#### `theme_dir` Configuration Option fully Deprecated
 
 As of version 0.17, the [custom_dir] option replaced the deprecated `theme_dir`
 option. If users had set the `theme_dir` option, MkDocs version 0.17 copied the
@@ -238,6 +238,7 @@ value to the `theme.custom_dir` option and a warning was issued. As of version
 
 ### Other Changes and Additions to Version 1.0
 
+* A missing theme configuration file now raises an error.
 * Empty `extra_css` and `extra_javascript` settings no longer raise a warning.
 * Add highlight.js configuration settings to built-in themes (#1284).
 * Close search modal when result is selected (#1527).

mkdocs/theme.py

@@ -83,12 +83,10 @@ class Theme(object):
                 theme_config = utils.yaml_load(f)
         except IOError as e:
             log.debug(e)
-            # TODO: Change this warning to an error in a future version
-            log.warning(
+            raise ValidationError(
                 "The theme '{0}' does not appear to have a configuration file. "
                 "Please upgrade to a current version of the theme.".format(name)
             )
-            return
 
         log.debug("Loaded theme configuration for '%s' from '%s': %s", name, file_path, theme_config)