I did this on the Simplicity, and Venus themes. I don't know how many others may have the heart icon in the newsfeed. I also went ahead and changed the comment Icon for the newsfeed as well so they would match. I have attached details on how I did this, and screen shots of how they look. I've seen a few people asking about this on the theme posts. I would just attach my svg file, but it won't let me attach an svg.
You will need to replace the file on each theme update. I couldn't find
any way of doing this through custom css in the admin panel. There may
be some script dependencies that are specifically calling for the
miniic_set.svg file.