You should be able to override almost any mustache template for any plugin including forum by adding a modified copy of the original file into your theme. For more information, see:
https://docs.moodle.org/dev/Templates#How_to_I_override_a_template_in_my_theme.3F
That way you don't need to modify any core code. If you are using a core theme such as Boost for your Moodle site, consider creating a child theme and using it instead with Boost as the parent. It will look exactly the same except that you will be able to modify it without affecting core code. For information on creating a child theme based on Boost, see:
https://docs.moodle.org/dev/Creating_a_theme_based_on_boost
Hope you find this helpful.
Best regards,
Michael Milette
https://docs.moodle.org/dev/Templates#How_to_I_override_a_template_in_my_theme.3F
That way you don't need to modify any core code. If you are using a core theme such as Boost for your Moodle site, consider creating a child theme and using it instead with Boost as the parent. It will look exactly the same except that you will be able to modify it without affecting core code. For information on creating a child theme based on Boost, see:
https://docs.moodle.org/dev/Creating_a_theme_based_on_boost
Hope you find this helpful.
Best regards,
Michael Milette