prompt: We are currently solving the following issue within our repository. Here is the issue text:
  --- BEGIN ISSUE ---
  Support built-in vega themes
  See [vega-themes](https://github.com/vega/vega-themes). Themes should be supported via the current theme infrastructure, maybe something like this:
  ```python
  alt.themes.enable('vega.themes.dark')
  ```
  We'll have to think about how to best populate the list of available themes, and how to make this work cleanly with user-specified themes from within Altair.
  
  --- END ISSUE ---
  
  Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
  --- BEGIN FILES ---
  Path: `altair/vegalite/v3/theme.py`
  Content:
  ```
  1 """Tools for enabling and registering chart themes"""
  2 
  3 from ...utils.theme import ThemeRegistry
  4 
  5 # The entry point group that can be used by other packages to declare other
  6 # renderers that will be auto-detected. Explicit registration is also
  7 # allowed by the PluginRegistery API.
  8 ENTRY_POINT_GROUP = 'altair.vegalite.v3.theme'  # type: str
  9 themes = ThemeRegistry(entry_point_group=ENTRY_POINT_GROUP)
  10 
  11 themes.register('default', lambda: {"config": {"view": {"width": 400, "height": 300},
  12                                                "mark": {"tooltip": None}}})
  13 themes.register('opaque', lambda: {"config": {"background": "white",
  14                                               "view": {"width": 400, "height": 300},
  15                                               "mark": {"tooltip": None}}})
  16 themes.register('none', lambda: {})
  17 themes.enable('default')
  18
  ```
  --- END FILES ---
  
  Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.
  
  Here is an example:
  
  ```diff
  diff --git a/examples/server_async.py b/examples/server_async.py
  --- a/examples/server_async.py
  +++ b/examples/server_async.py
  @@ -313,4 +313,4 @@
   
   
   if __name__ == "__main__":
  -    asyncio.run(run_async_server("."), debug=True)
  +    asyncio.run(run_async_server(), debug=True)
  diff --git a/examples/server_sync.py b/examples/server_sync.py
  --- a/examples/server_sync.py
  +++ b/examples/server_sync.py
  @@ -313,5 +313,5 @@
   
   
   if __name__ == "__main__":
  -    server = run_sync_server(".")
  +    server = run_sync_server()
       server.shutdown()
  
  ```
  
golden_diff: diff --git a/altair/vegalite/v3/theme.py b/altair/vegalite/v3/theme.py
  --- a/altair/vegalite/v3/theme.py
  +++ b/altair/vegalite/v3/theme.py
  @@ -2,6 +2,23 @@
   
   from ...utils.theme import ThemeRegistry
   
  +VEGA_THEMES = ['ggplot2', 'quartz', 'vox', 'fivethirtyeight', 'dark', 'latimes']
  +
  +
  +class VegaTheme(object):
  +    """Implementation of a builtin vega theme."""
  +    def __init__(self, theme):
  +        self.theme = theme
  +        
  +    def __call__(self):
  +        return {"usermeta": {"embedOptions": {"theme": self.theme}},
  +                "config": {"view": {"width": 400, "height": 300},
  +                           "mark": {"tooltip": None}}}
  +
  +    def __repr__(self):
  +        return "VegaTheme({!r})".format(self.theme)
  +
  +
   # The entry point group that can be used by other packages to declare other
   # renderers that will be auto-detected. Explicit registration is also
   # allowed by the PluginRegistery API.
  @@ -14,4 +31,8 @@
                                                 "view": {"width": 400, "height": 300},
                                                 "mark": {"tooltip": None}}})
   themes.register('none', lambda: {})
  +  
  +for theme in VEGA_THEMES:
  +    themes.register(theme, VegaTheme(theme))
  +
   themes.enable('default')
  
verification_info: {"golden_diff": "diff --git a/altair/vegalite/v3/theme.py b/altair/vegalite/v3/theme.py\n--- a/altair/vegalite/v3/theme.py\n+++ b/altair/vegalite/v3/theme.py\n@@ -2,6 +2,23 @@\n \n from ...utils.theme import ThemeRegistry\n \n+VEGA_THEMES = ['ggplot2', 'quartz', 'vox', 'fivethirtyeight', 'dark', 'latimes']\n+\n+\n+class VegaTheme(object):\n+    \"\"\"Implementation of a builtin vega theme.\"\"\"\n+    def __init__(self, theme):\n+        self.theme = theme\n+        \n+    def __call__(self):\n+        return {\"usermeta\": {\"embedOptions\": {\"theme\": self.theme}},\n+                \"config\": {\"view\": {\"width\": 400, \"height\": 300},\n+                           \"mark\": {\"tooltip\": None}}}\n+\n+    def __repr__(self):\n+        return \"VegaTheme({!r})\".format(self.theme)\n+\n+\n # The entry point group that can be used by other packages to declare other\n # renderers that will be auto-detected. Explicit registration is also\n # allowed by the PluginRegistery API.\n@@ -14,4 +31,8 @@\n                                               \"view\": {\"width\": 400, \"height\": 300},\n                                               \"mark\": {\"tooltip\": None}}})\n themes.register('none', lambda: {})\n+  \n+for theme in VEGA_THEMES:\n+    themes.register(theme, VegaTheme(theme))\n+\n themes.enable('default')\n", "issue": "Support built-in vega themes\nSee [vega-themes](https://github.com/vega/vega-themes). Themes should be supported via the current theme infrastructure, maybe something like this:\r\n```python\r\nalt.themes.enable('vega.themes.dark')\r\n```\r\nWe'll have to think about how to best populate the list of available themes, and how to make this work cleanly with user-specified themes from within Altair.\n", "before_files": [{"content": "\"\"\"Tools for enabling and registering chart themes\"\"\"\n\nfrom ...utils.theme import ThemeRegistry\n\n# The entry point group that can be used by other packages to declare other\n# renderers that will be auto-detected. Explicit registration is also\n# allowed by the PluginRegistery API.\nENTRY_POINT_GROUP = 'altair.vegalite.v3.theme'  # type: str\nthemes = ThemeRegistry(entry_point_group=ENTRY_POINT_GROUP)\n\nthemes.register('default', lambda: {\"config\": {\"view\": {\"width\": 400, \"height\": 300},\n                                               \"mark\": {\"tooltip\": None}}})\nthemes.register('opaque', lambda: {\"config\": {\"background\": \"white\",\n                                              \"view\": {\"width\": 400, \"height\": 300},\n                                              \"mark\": {\"tooltip\": None}}})\nthemes.register('none', lambda: {})\nthemes.enable('default')\n", "path": "altair/vegalite/v3/theme.py"}], "after_files": [{"content": "\"\"\"Tools for enabling and registering chart themes\"\"\"\n\nfrom ...utils.theme import ThemeRegistry\n\nVEGA_THEMES = ['ggplot2', 'quartz', 'vox', 'fivethirtyeight', 'dark', 'latimes']\n\n\nclass VegaTheme(object):\n    \"\"\"Implementation of a builtin vega theme.\"\"\"\n    def __init__(self, theme):\n        self.theme = theme\n        \n    def __call__(self):\n        return {\"usermeta\": {\"embedOptions\": {\"theme\": self.theme}},\n                \"config\": {\"view\": {\"width\": 400, \"height\": 300},\n                           \"mark\": {\"tooltip\": None}}}\n\n    def __repr__(self):\n        return \"VegaTheme({!r})\".format(self.theme)\n\n\n# The entry point group that can be used by other packages to declare other\n# renderers that will be auto-detected. Explicit registration is also\n# allowed by the PluginRegistery API.\nENTRY_POINT_GROUP = 'altair.vegalite.v3.theme'  # type: str\nthemes = ThemeRegistry(entry_point_group=ENTRY_POINT_GROUP)\n\nthemes.register('default', lambda: {\"config\": {\"view\": {\"width\": 400, \"height\": 300},\n                                               \"mark\": {\"tooltip\": None}}})\nthemes.register('opaque', lambda: {\"config\": {\"background\": \"white\",\n                                              \"view\": {\"width\": 400, \"height\": 300},\n                                              \"mark\": {\"tooltip\": None}}})\nthemes.register('none', lambda: {})\n  \nfor theme in VEGA_THEMES:\n    themes.register(theme, VegaTheme(theme))\n\nthemes.enable('default')\n", "path": "altair/vegalite/v3/theme.py"}]}