Add hljs_style config to mkdocs theme.

This is primarily to allow third party themes which inherit from the 
mkdocs theme (such as the bootswatch themes) to alter the code 
highlighting style. Previously all of the themes hosted their own 
highlighting styles, but now that we are using the CDN, it is easier to 
use a setting, which downstream themes can override.

docs/user-guide/styling-your-docs.md

@@ -36,6 +36,10 @@ supports the following options:
 * __`highlightjs`__: Enables highlighting of source code in code blocks using
   the [highlight.js] JavaScript library. Default: `True`.
 
+* __`hljs_style`__: The highlight.js library provides 79 different [styles]
+  (color variations) for highlighting source code in code blocks. Set this to
+  the name of the desired style. Default: `github`.
+
 * __`hljs_languages`__: By default, highlight.js only supports 23 common
   languages. List additional languages here to include support for them.
 
@@ -69,12 +73,18 @@ supports the following options:
 
     * __`search`__: Display the search modal. Default: `83` (s)
 
+[styles]: https://highlightjs.org/static/demo/
+
 ### readthedocs
 
 A clone of the default theme used by the [Read the Docs] service, which offers
 the same restricted feature-set as its parent theme. Like its parent theme, only
-two levels of navigation are supported. In addition to the default [theme
-configuration options], the `readthedocs` theme supports the following options:
+two levels of navigation are supported.
+
+![ReadTheDocs](../img/readthedocs.png)
+
+In addition to the default [theme configuration options], the `readthedocs`
+theme supports the following options:
 
 * __`highlightjs`__: Enables highlighting of source code in code blocks using
   the [highlight.js] JavaScript library. Default: `True`.
@@ -82,16 +92,12 @@ configuration options], the `readthedocs` theme supports the following options:
 * __`hljs_languages`__: By default, highlight.js only supports 23 common
   languages. List additional languages here to include support for them.
 
-```yaml
-theme:
-    name: readthedocs
-    highlightjs: true
-    hljs_languages:
-        - yaml
-        - rust
-```
-
-![ReadTheDocs](../img/readthedocs.png)
+        theme:
+            name: readthedocs
+            highlightjs: true
+            hljs_languages:
+                - yaml
+                - rust
 
 ### Third Party Themes
 

mkdocs/tests/config/config_tests.py

@@ -130,6 +130,7 @@ class ConfigTests(unittest.TestCase):
                         'include_search_page': False,
                         'search_index_only': False,
                         'highlightjs': True,
+                        'hljs_style': 'github',
                         'hljs_languages': [],
                         'shortcuts': {'help': 191, 'next': 78, 'previous': 80, 'search': 83}
                     }
@@ -173,6 +174,7 @@ class ConfigTests(unittest.TestCase):
                         'include_search_page': False,
                         'search_index_only': False,
                         'highlightjs': True,
+                        'hljs_style': 'github',
                         'hljs_languages': [],
                         'shortcuts': {'help': 191, 'next': 78, 'previous': 80, 'search': 83}
                     }

mkdocs/tests/theme_tests.py

@@ -32,6 +32,7 @@ class ThemeTests(unittest.TestCase):
             'include_search_page': False,
             'search_index_only': False,
             'highlightjs': True,
+            'hljs_style': 'github',
             'hljs_languages': [],
             'shortcuts': {'help': 191, 'next': 78, 'previous': 80, 'search': 83}
         })

mkdocs/themes/mkdocs/base.html

@@ -21,7 +21,7 @@
         <link href="{{ 'css/font-awesome.min.css'|url }}" rel="stylesheet">
         <link href="{{ 'css/base.css'|url }}" rel="stylesheet">
         {%- if config.theme.highlightjs %}
-        <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/github.min.css">
+        <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/{{ config.theme.hljs_style }}.min.css">
         {%- endif %}
         {%- for path in config['extra_css'] %}
         <link href="{{ path|url }}" rel="stylesheet">

mkdocs/themes/mkdocs/mkdocs_theme.yml

@@ -8,6 +8,7 @@ search_index_only: false
 
 highlightjs: true
 hljs_languages: []
+hljs_style: github
 
 shortcuts:
     help: 191    # ?