If the text contains keywords for multiple themes, I suspect that all of the matching themes should be tagged. Isn’t that why it is a multiple-select instead of a single-select?
I think that a linked record field would be better than a multi-select field. Storing the themes in their own table would automatically give a place to store the related keywords. Then an automation script can search the text for keywords and make the record links based on the matches found. Since the primary reason for tagging themes is for finding all the records under the same theme, it would be easier to identify those records in the linked record field of the theme.
