Wow, thats great. I mean, I can see that there is a lot of exciting options ahead for us and i trust that the UI / UX team has a good judgement on this. I think you can silence most sceptics of the new change with the message that they will be able to either have options to adjust colors or options to choose from themes.
If i where in your team i would probably say “i am fine if someone wants to choose really weird color combinations” - so i would give the ability to choose colors for certain areas like the sidebar. But i also understand the counter argument and the ask for more control on HubSpots side, which would then call for the selection of themes that are provided by Hubspot.