It looks like a polished stone surface to me. Download it for free, as always.
Source V. Hartikainen
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
An interesting dark spotted pattern at an angle.
Source Hendrik Lammers
Three shades of gray makes this pattern look like a small carbon fiber surface. Great readability even for small fonts.
Source Atle Mo
A free seamless background texture of "timber wall" (colored in dark brown).
Source V. Hartikainen
A seamless pattern formed from a square tile. To get the tile, 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
There are many carbon patterns, but this one is tiny.
Source Designova
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Remixed from a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Prismatic Snowflakes Pattern 3 No Background
Source GDJ
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
Thin lines, noise and texture creates this crisp dark denim pattern.
Source Marco Slooten
Here's a seamless brown cork board background texture. Feel free to download or reshare if you like.
Source V. Hartikainen
Psychedelic Geometric Background No Black
Source GDJ
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i
Source Firkin
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
Remixed from a design seen on Pixabay. The basic tile can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Number 3 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
It’s like Shine Dotted’s sister, only rotated 45 degrees.
Source mediumidee
Prismatic Curved Diamond Pattern 4 No Background
Source GDJ
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin