Hi,
Some component builders allow site builders to create their own themes by checking for a folder templates/my-template/html/com_jsn/assets/css/ and then loading the css from there rather than from /comopnents/lcom_jsn/assets/css/
Joomla natively allow override only of PHP views files, but maybe in future we can implement something like this.
One more thing, I wonder why we still use bootstrap classes when we cave css display:flex and css display:grid?
It's simple, Joomla uses bootstrap classes and have APIs to load own bootstrap framework. So, all Joomla templates already have a Bootstrap CSS style included. This allow us to not put CSS to reset and restyle entire output to fit alla templates. Keep in mind that our component works for all Joomla 3.x versions (Joomla 3.0 use bootstrap 2) and it need to fit all templates(bootstrap 2 and 3 and in future also 4), so it's normal to use !important statement in some cases.
Of course the CSS code could be improved in future.