Allow passing theme to Markdown converter

Use on code blocks

lib/beacon/template/markdown.ex

@@ -7,8 +7,18 @@ defmodule Beacon.Template.Markdown do
 
   # TODO: implement a markdown format that is aware of Phoenix features like link attrs and assigns
 
+  @doc """
+  Convert a markdown `template` into HTML using [mdex](https://hex.pm/packages/mdex)
+
+  ## Options
+
+    * `:syntax_highlight_theme` (default `"onedark"`) - see https://hexdocs.pm/mdex/MDEx.html#to_html/2 for more info.
+
+  """
   @spec convert_to_html(Beacon.Template.t(), Beacon.Template.LoadMetadata.t()) :: {:cont, Beacon.Template.t()} | {:halt, Exception.t()}
-  def convert_to_html(template, _metadata) do
+  def convert_to_html(template, _metadata, opts \\ []) do
+    syntax_highlight_theme = Keyword.get(opts, :syntax_highlight_theme, "onedark")
+
     template =
       MDEx.to_html(template,
         extension: [
@@ -26,7 +36,7 @@ defmodule Beacon.Template.Markdown do
           unsafe_: true
         ],
         features: [
-          syntax_highlight_theme: "onedark"
+          syntax_highlight_theme: syntax_highlight_theme
         ]
       )