I understand that.  But Joomla likes to replace all files and folder structure on upgrades (mainly the component files).  This answer is based on limited knowledge about Matias concerns about adding items like this.  Basically the answer I am forwarding is Joomla upgrades are weird sometimes and we need to confirm what is removed and not removed.

Problem is it is not tested.  What I want to avoid is someone creating a custom CSS file, adding all the changes, performing an upgrade through Joomla, and that file going missing.  I agree the change is trivial, but even the trivial change might create a scenario down the road that could have a negative side effect.