Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
A free green background pattern with a pattern of rhombuses on a seamless texture. Feel free to use it as a tiled background image on your web site.
Source V. Hartikainen
A playful triangle pattern with different shades of gray.
Source Dimitrie Hoekstra
The following repeating website background is colored in a blue gray color and resembles a concrete wall or something similar to it.
Source V. Hartikainen
Alternative colour scheme for the original floral pattern.
Source Firkin
After 1 comes 2, same but different. You get the idea.
Source Hendrik Lammers
Remixed from a raster on Pixabay, that was uploaded by ArtsyBee.
Source Firkin
This seamless pattern consists of a blue grid on a yellow background.
Source V. Hartikainen
Remixed from a drawing in 'The March of Loyalty', Letitia MacClintock, 1884.
Source Firkin
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
From a drawing in 'Gately's World's Progress', Charles Beale, 1886.
Source Firkin
Derived from a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
The rectangular tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
A simple example on using clones. You can generate a nice base for a pattern fill quickly with it.
Source Lazur URH
CC0 remixed from a drawing. Walter Crane, 1914, Firkin.
Source SliverKnight
High detail stone wall with minor cracks and specks.
Source Projecteightyfive
A bit strange this one, but nice at the same time.
Source Diogo Silva
Heavy depth and shadows here, but might work well on some mobile apps.
Source Damian Rivas
A large pattern with funky shapes and form. An original. Sort of origami-ish.
Source Luuk van Baars
Could be paper, could be a Polaroid frame – up to you!
Source Chaos
Simple combination of stripy squares with their negatively coloured counterparts
Source Firkin