Heavy depth and shadows here, but might work well on some mobile apps.
Source Damian Rivas
There are many carbon patterns, but this one is tiny.
Source Designova
The image depicts meshed silhouettes of various things.The original image is an OCAL clipart called "Enter FOSSASIA 2016 #IoT T-shirt Design Contest" uploaded by "openclipart".Thanks.
Source Yamachem
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
Super subtle indeed, a medium gray pattern with tiny dots in a grid.
Source Designova
From a drawing in 'Less Black than we're painted', James Payn, 1884.
Source Firkin
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
This was formed by distorting an image of a background on Pixabay.
Source Firkin
A seamless chequerboard pattern formed from a tile that can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
Prismatic Isometric Cube Extra Pattern No Background
Source GDJ
Recreated from a pattern found in 'Az Osztrák-Magyar Monarchia irásban és képben', 1882. To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Medium gray pattern with small strokes to give a weave effect.
Source Catherine
Could be paper, could be a Polaroid frame – up to you!
Source Chaos
Remixed from a drawing in 'A Girl in Ten Thousand', Elizabeth Meade, 1896.
Source Firkin
From a drawing in 'Gately's World's Progress', Charles Beale, 1886.
Source Firkin
I have no idea how to describe this one, but it’s light and delicate.
Source JBasoo
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
Medium gray pattern with small strokes to give a weave effect.
Source Catherine
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
From a drawing in 'Handbook of the excursions proposed to be made by the Lincoln Diocesan Architectural Society, on the 27th and 28th of May, 1857', Edward Trollope, 1857.
Source Firkin
No relation to the band, but damn it’s subtle!
Source Thomas Myrman
A seamless pattern with green and yellow diagonal lines on top of a white dotted background.
Source V. Hartikainen