Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Never out of fashion and so much hotter than the 45º everyone knows, here is a sweet 60º line pattern.
Source Atle Mo
A light brushed aluminum pattern for your pleasure.
Source Tim Ward
A comeback for you: the popular Escheresque, now in black.
Source Patten
Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-i
Source Firkin
Inspired by a design found in 'Konstantinápolyi emlékeim', Miklos Chriszto, 1893.
Source Firkin
Tiny little fibers making a soft and sweet look.
Source Badhon Ebrahim
Prismatic Geometric Pattern Background No Black
Source GDJ
The starting point for this was a texture drawn with the 'Radial Colors' plug-in in Paint.net.
Source Firkin
Prismatic Abstract Geometric Background 2
Source GDJ
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
ZeroCC tileable stone texture, edited from pixabay, CC0
Source Sojan Janso
Nothing like a clean set of bed sheets, huh?
Source Badhon Ebrahim
Not sure if this is related to the Nami you get in Google image search, but hey, it’s nice!
Source Dertig Media
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
That’s what it is, a dark dot. Or sort of carbon looking.
Source Tsvetelin Nikolov
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin