I would disagree. Custom themes will make it easy for non code people to change the Folders to their taste to make it look like the view you have.
Secondly - The vast majority of people, myself included will be using the folders directly and saving the attachments into the folders area within Manager. I have all my attachments within Manager already. There is no benefit to me to host my attachments on a third party platform.
So a custom theme is still very relevant.
What you are actually requesting is that the plain theme for folders is modified so that it does not show the headers. It would help if you could clarify what headers you are actually referring to? The name folder and your company logo or what?
With my custom theme that I use for Invoices etc - I have my address on a single line at the bottom of the invoice page. This takes up significantly less space than having the address going down several lines on the RHS. With a custom theme for folders, you can simply delete the company address information as it’s not really relevant to folders.
In short, Folders actually needs a plain theme specifically designed for Folders very similar to what you have, but without the logo and address. Then custom themes to give users the ability to easily customise the colours as you have done.