Thanks for the update and sorry about the misunderstanding. It would seem that there is no easy way to do this for the MacOS version of the app. However, I have found a couple of links for you to check and see if anything there helps you.
The first is a link from Microsoft Support on the Windows version of the app. Perhaps the instructions there would be valid for your version of the app as well. https://support.microsoft.com/office/customize-or-create-new-styles-d38d6e47-f6fc-48eb-a607-1eb120dec563
The second is a link on how to access the color scheme via an alternate method. https://www.brandwares.com/bestpractices/2015/05/xml-hacking-color-themes/
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.