Hi, Brontosuchus
Thank you for using Microsoft products and posting them to the community.
It looks like you want to make sure that all the shapes in your Word document automatically adjust their colors when you switch between dark and light modes. Unfortunately, Word does not currently have a built-in feature to automatically reverse the colors of shapes drawn with the mouse.
You might consider using a macro: If you're familiar with VBA (Visual Basic for Applications), you can create a macro that changes the color of a shape based on the current theme. This requires some coding, but it is possible to automate the process.
But this aspect is beyond the support of the Microsoft forums, and you can go to this platform for help: Newest 'vba' Questions - Stack Overflow
Also, you can give feedback to Microsoft: if this feature is important to you, consider providing feedback to Microsoft. They will update the software according to the user's needs.
I hope the above information can help you. Feel free to send a message if you need further help.
Best wishes
Aiden.C - MSFT |Microsoft Community Support Specialist