Add program argument for syntax highlighting theme.

The source code output in the chat will be colored in the given
color scheme. Syntax highlighting is powered by pygments, which
supports many styles out of the box. For example:

- default (the default)
- dracula
- emacs
- github-dark
- monokai
- vim
- solarized-dark
- solarized-light

See https://pygments.org/styles/

aider/coders/base_coder.py

@@ -111,6 +111,7 @@ class Coder:
         map_tokens=1024,
         verbose=False,
         assistant_output_color="blue",
+        code_theme="default",
         stream=True,
         use_git=True,
     ):
@@ -135,6 +136,7 @@ class Coder:
         self.auto_commits = auto_commits
         self.dirty_commits = dirty_commits
         self.assistant_output_color = assistant_output_color
+        self.code_theme = code_theme
 
         self.dry_run = dry_run
         self.pretty = pretty
@@ -679,7 +681,7 @@ class Coder:
 
         show_resp = self.render_incremental_response(True)
         if self.pretty:
-            show_resp = Markdown(show_resp, style=self.assistant_output_color, code_theme="default")
+            show_resp = Markdown(show_resp, style=self.assistant_output_color, code_theme=self.code_theme)
         else:
             show_resp = Text(show_resp or "<no response>")
 
@@ -735,7 +737,7 @@ class Coder:
         if not show_resp:
             return
 
-        md = Markdown(show_resp, style=self.assistant_output_color, code_theme="default")
+        md = Markdown(show_resp, style=self.assistant_output_color, code_theme=self.code_theme)
         live.update(md)
 
     def render_incremental_response(self, final):

aider/main.py

@@ -145,6 +145,11 @@ def main(args=None, input=None, output=None):
         default="blue",
         help="Set the color for assistant output (default: blue)",
     )
+    parser.add_argument(
+        "--code-theme",
+        default="default",
+        help="Set the markdown code theme (default: default, other options include monokai, solarized-dark, solarized-light)",
+    )
     parser.add_argument(
         "--apply",
         metavar="FILE",
@@ -272,6 +277,7 @@ def main(args=None, input=None, output=None):
         map_tokens=args.map_tokens,
         verbose=args.verbose,
         assistant_output_color=args.assistant_output_color,
+        code_theme=args.code_theme,
         stream=args.stream,
         use_git=args.git,
     )