Frameless would be the way I'd head, as that seems to be where Adobe is making most of their changes.
I'd recommend using the editor to modify the skin settings that are available, then add a custom css to the User Assets section of the skin for anything that's not available in the editor. Otherwise I think you risk breaking the skin file.
For the topic background, perhaps look in the skin editor at Topic Page > Center Panel > container and topic-box. (The wording might be slightly different in 2020 - I only have 2019 at the moment but the general area should be right)
If you mean changing everything that's white, then there'll be a few places. For example Body, Header, Left Panel, etc. The highlight should help you determine what will change (make sure the Toggle Highlight button is selected at the top of the application)