How can I set dark mode for Word documents
The easiest way is to create a new template file:

File -&gt; New Document


click on Design tab then click on Page Color icon and select More Colors.
There, you can use a color picker to sample you favorite color (I picked #282a36 in the Dracula theme in BBEdit) and set this color as your page color. 
And after that, select

File -&gt; Save as Template


and then Save. 
From now on, when you open Word, select

File -&gt; New from Template


and select your template.