Unfortunately, the only option you have is to turn back to the classic experience at this moment. Right now, Microsoft doesn't allow any branding, theming or customization on the "new list/library experience".
There is a request in Uservoice related to this and MS has given a positive response to it. So, in the future, no timeline defined, we will be able to customize it.
To enable classic experience for all users in the tenant, you can set it from tenant settings as below:
Go to the portal home > SharePoint > settings
To set it at individual list/doc lib, you can go to the list settings page > advanced settings > classic experience.