Leftover tables - how to clean after uninstall?

Problem/Motivation

I believe, several months ago, I've tested electra briefly and uninstalled it again.
There are no folders / files anymore and I've used the standard uninstall method via the admin first.
However, I just found these tables (empty) in my DB:

block_content__field_electra_color_type
block_content__field_electra_parallax_bg_image
block_content__field_electra_parallax_bg_type
block_content__field_electra_parallax_type

How to uninstall the theme in a clean manner, after uninstalling it via the proper way?

Steps to reproduce

Not sure - probably install and uninstall?