Custom Modifier isn't found by server in `style` tag.

[BrooklinJazz]

@carson-katri and I defined a custom modifier called markdownTheme for a wrapper around the swift-markdown-ui library.

It doesn't seem to appear in the stylesheet when applied through the style attribute, but it does appear in a class.

This doesn't work:

<Markdown style="markdownTheme(.gitHub)" baseURL="https://google.com">

But this does:

~SHEET"""
"my-style" do
  markdownTheme(.gitHub)
end
"""

<Markdown style="markdownTheme(.gitHub)" class="my-style" baseURL="https://google.com">

The stylesheet output file is empty when using the style attribute. But contains the following when using class and a stylesheet:

%{"my-style" => [{:markdownTheme, [], [{:., [], [nil, :gitHub]}]}]}