This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud
This pack of filters can help you adding a blocky overlay to objects. May come handy at drawing blocks of stone.
Source Lazur URH
Prismatic Triangular Background Design Mark II 5
Source GDJ
Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
A seamless pattern created from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A seamless pattern from a tile made from a jpg on Pixabay. To get the tile select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Zero CC tileable hard cover cells book texture, 4k, scanned and made by me. CC0
Source Sojan Janso
Prismatic Groovy Concentric Background
Source GDJ
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
A new take on the black linen pattern. Softer this time.
Source Atle Mo
Remixed from a drawing in 'Line and form", Walter Crane, 1914.
Source Firkin
Prismatic Rounded Squares Grid 3 No Background
Source GDJ
Remixed from an image that was uploaded to Pixabay by CatherineClennan
Source Firkin
A dark background pattern/texture of a dimpled metal plate.
Source V. Hartikainen
Non-seamless pattern drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
Here's a quite bright pink background pattern for use on websites. It doesn't look like a real fur, but it definitely resembles one.
Source V. Hartikainen
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
A pattern drawn in Paint.net and vectorized in Inkscape.
Source Firkin
An abstract pale yellow paper-like background with stains colored in yellow and green.
Source V. Hartikainen
This one needs to be used in small areas; you can see it repeat.
Source Luca
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Classy golf-pants pattern, or crossed stripes if you will.
Source Will Monson
Feel free to use this seamless background texture as a background on a web site. It's colored in a light pink color and is seamlessly tile-able.
Source V. Hartikainen
Abstract Arbitrary Geometric Background derived from an image on Pixabay.
Source GDJ