you would have to edit the css in the style files that come with the extension. the same way you have been doing this for the rest of your site.
however, the extensions do not use the files from standard phpbb, they are self contained. therefore, you have to actually edit the files in the relevant style folder of the extension.
in this case, it would be here:
you can still use the inspect element system to find exactly which line a given font is controlled by in that css file.
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