First, please post your "bonus" question as a separate question. You'll get better answers.


There are two separate themes involved in Word and Office.


The first is the Word Theme that applies to elements in a document. (Also can be used in other Office applications) The settings for this, in recent versions, are under the Design tab.

The second is the Office Display theme which applies to how applications appear in Windows. The settings for this are under File > Account.


The first can be customized; you can create your own themes.
The second you only have the choices offered by Microsoft. That was equally true with Office 2016.


[Quick] Style Sets and Word Themes in Microsoft Word