I want to remove the top bars and the bottom as well, so that I pretty much have a blank page.
It is not possible without using the SharePoint Framework, and even then its not recommended/supported. It's not recommended because the pace of change of Office 365 means that any DOM manipulation of OOB code is likely to be broken at any time by updates from Microsoft.
That said, there are 3 ways off the top of my head that you can do this, if you choose to proceed at your own risk:
The last two work for customizing a single page. The first will do all modern pages. These techniques may be limited depending on whether the site is marked no script. And all of them require that you have sufficient privileges to deploy Spfx solutions.