it's weird that. it's obviously possible to have a flat-shaded skeuomorph, just look at basically all of windows 95, but for some reason we connect them to this particular graphical style. files and folders are both part of the old classic "desktop metaphor", so they basically have to be skeuomorphs. but like, the application icons are basically just mosaic tiles of the normal icons.
a proper skeuomorph would indicate what the program is for. krita and whatever map software that is are both good, if a little flat. but the libreoffice suite just being squares with a letter on them? have them be like, a spreadsheet for calc, a stack of cards for impress, and a printed page for write.
remember all the icons for windows 95 network utilities that have people in them? those are also (attempts at) skeumorphs because they're trying to communicate what the program does.
i use a Kobo Clara HD. It runs linux out of the box, the system memory is on a removable SD card inside the case, and the user account is defined in an SQLite database on disk. If you add an empty user account to the database, it removes the "create account" screen and disables any Kobo online services. Then you can install KOReader and upload files over USB as everyone else has said.