Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Prismatic Isometric Cube Extra Pattern No Background
Source GDJ
Detailed but still subtle and quite original. Lovely gray shades.
Source Kim Ruddock
Prismatic Abstract Geometric Background 2
Source GDJ
Thin lines, noise and texture creates this crisp dark denim pattern.
Source Marco Slooten
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868.
Source Firkin
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
A mid-tone gray pattern with some cement looking texture.
Source Hendrik Lammers
Prismatic Groovy Concentric Background 4 No Black
Source GDJ
Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
Little x’es, noise and all the stuff you like. Dark like a Monday, with a hint of blue.
Source Tom McArdle
Prismatic Geometric Pattern Background No Black
Source GDJ
Number 4 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
This was formed by distorting an image of a background on Pixabay.
Source Firkin
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
From a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
Alternative colour scheme. Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
A pattern formed from a squared tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
From a drawing in 'Hyde Park from Domesday-Book to date', John Ashton, 1896.
Source Firkin
This is the remix of an OCAL clipart called "Art Nouveau ornament" uploaded by "microcosme".Thanks.This is a seamless pattern of an Art Nouveau ornament.
Source Yamachem