Hi jaxxxx4,
>> So, in the home tab of a word document. There is this nifty thing to change the 'style' of your document. I chose "office" theme. and now my header in my table looks red, right? cool.
What's the version of Word you are using?
Are you referring to the Themes group in Design ribbon as follows:
If this is the case, as far as I know, this is the expected behavior that themes apply to the entire document.
In your situation, you may consider using the Table Styles instead to assign styles separately to different tables:
https://support.office.com/en-us/article/apply-table-styles-0f19e350-f9a1-4a9f-afdb-46ee2bb8460c

Hope this can be helpful.
Regards,
Yuki Sun
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