      >  I want to create custom styles, but it seems the only ones that are available are the Headers.

First, each component in Markdown will be translated in a docx component with a default style apply to them. Best way to see what component (code, list, header, ...) is associated to what style is to add the component in a document, and then compile to docx and look in the resulting document which style is applied. You can use this document as based for tweaking and provide as non default reference doc.

If I understand correctly your use case is also creating new styles from scratch in your template docx document, and then you want to be able to use them (apply them) from your source `.qmd` file. 

If that is the case, you can do that for DOCX document using Custom style feature from Pandoc: https://pandoc.org/MANUAL.html#custom-styles. It appears we don't have mention to that in our doc and we should probably add it. 

Syntax would be for inline style 
````
[Get out]{custom-style="Emphatically"}, he said.
````
and 
````markdown
::: {custom-style="Poetry"}
| A Bird came down the Walk---
| He did not know I saw---
:::
````
for paragraph style

You can also reference existing style and not just new one. 

Hope it helps

_Originally posted by @cderv in https://github.com/quarto-dev/quarto-cli/discussions/2657#discussioncomment-3763204_
    
2022-09-29T14:09:11Z
cderv
documentation
v1.4
open
Document custom styles syntax for using with DOCX template
https://github.com/quarto-dev/quarto-cli/issues/4502