Removing user defined formatting from a PBIX file

Hi All,

I have developed a  PowerBI Theme file (JSON file) for our company. This defines the company colors, font sizes etc.

We need to retrospectively apply this theme to a large number of existing PBI reports that don't meet the company's style guidelines. 

The problem with these old reports, is that importing a JSON theme file, doesn't over ride user defined formatting. This is expected behavior as outlined here : https://docs.microsoft.com/en-us/power-bi/desktop-report-themes#situations-when-report-theme-colors-wont-stick-to-your-reports

 Therefore we are forced to manually reset these user defined formatting- which is time consuming. 

Can anyone suggest a way to remove user defined formatting, so that the Theme formatting can be effeciently applied?

As a heads up - I have tried digging into the  unzipped PBI files and deleting out existing JSON files in the "Base Themes folder- but user defined formatting don't appear to be controlled by JSON files. 

Any help would be appreciated.

Cheers

Steve