fix: mkdocstrings unable to parse custom theme directory configuration

This fixes the mkdocs-material's custom_dir configuration, where theme `name` is set to null in mkdocs.yaml.
Thereby, this fix adds support to automatically read theme_name from `dirs` attribute, if `name` attribute is None(null), otherwise default behaviour will occur.

Fixes mkdocstrings#120.

src/mkdocstrings/plugin.py

@@ -25,6 +25,7 @@
 import logging
 import random
 import re
+import os
 import string
 from typing import Any, Callable, Dict, List, Match, Optional, Pattern, Tuple
 
@@ -151,8 +152,14 @@ def on_config(self, config: Config, **kwargs) -> Config:
 
         log.debug("mkdocstrings.plugin: Adding extension to the list")
 
+        theme_name = None
+        if config["theme"].name is None:
+            theme_name = os.path.dirname(config["theme"].dirs[0])
+        else:
+            theme_name = config["theme"].name
+
         extension_config = {
-            "theme_name": config["theme"].name,
+            "theme_name": theme_name,
             "mdx": config["markdown_extensions"],
             "mdx_configs": config["mdx_configs"],
             "mkdocstrings": self.config,