Thanks for raising this - we have discovered this to be a bug when we transitioned from Symphony 1.5 to 2.0. One of the parity items that was missed out on. This has been added to the backlog for fixing but we don't have a specific date for when it will be done. Apologies that you will have to find an alternative icon that is neutral to both light and dark themes in the meantime.
Thanks for investigating and confirming that - much appreciated. Yes, we will use an alternative single icon for the time being.