Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Washi (和紙?) is a type of paper made in Japan. Here’s the pattern for you!
Source Carolynne
Prismatic Rounded Squares Grid 4 No Background
Source GDJ
Prismatic Floral Background No Black
Source GDJ
Your eyes can trip a bit from looking at this – use it wisely.
Source Michal Chovanec
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
From a drawing in 'An Old Maid's Love. A Dutch tale told in English', Maarten Maartens, 1891.
Source Firkin
Some rectangles, a bit of dust and grunge, plus a hint of concrete.
Source Atle Mo
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
I have no idea how to describe this one, but it’s light and delicate.
Source JBasoo
Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Sometimes you just need the simplest thing.
Source Fabricio
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
Zero CC tileable bark texture, photographed and made by me. CC0
Source Sojan Janso
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8