Hugo can process SCSS files which live in <site root>/assets or </site root>/theme/<name>/assets.
From what I understand of your post, I don’t think the theme uses Hugo to compile SCSS to CSS - it probably distributes the site with the CSS files, and is just including things in static/assets/plugin for completeness. (Files and directories under static are simply copied verbatim into the /public folder by Hugo.)
So what you probably want to do is override the compiled CSS files. (Have a read of the documentation about lookup order to understand how this works.)
So for example, put a copy of <site root>/themes/<themename>/static/assets/css/page.css in <site root>/static/assets/css/page.css and then edit your version of page.css to your liking.