Joomla Monster seem to have added their css in a non-standard way by uploading it to the
modules rather than
media folder. This means the usual css overrides will not work.
However, in the module, there is an option called "module theme". Set this to "Override from template" and it will remove all their default styles.
You can find their styles under
modules/mod_jm_articles_category/assets/default.css should you then want to copy these into your template's style sheet and edit them from there.
However, looking at their stylesheet, you should just be able to make small overrides in your template's style without replacing the whole thing - I suspect your previous edits might just need to be a bit more specific so that they take precedence over the module's default styles (or just whack in an !important if you are happy with lazy code).