Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Kaleidoscope Prismatic Abstract No Background
Source GDJ
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
One more brick pattern. A bit more depth to this one.
Source Benjamin Ward
Nothing like a clean set of bed sheets, huh?
Source Badhon Ebrahim
This is lovely, just the right amount of subtle noise, lines and textures.
Source Richard Tabor
Prismatic Polka Dots 3 No Background
Source GDJ
Spice up your next school project with this icon background.
Source Swetha
Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
Seamless Prismatic Pythagorean Line Art Pattern No Background. A seamless pattern that includes the original tile (go to Objects / Pattern / Pattern To Objects in Inkscape's menu to extract it).
Source GDJ
Abstract Stars Geometric Pattern Prismatic No Background
Source GDJ
Paper pattern with small dust particles and 45-degree strokes.
Source Atle Mo
Seamless Light Background Texture.
Source V. Hartikainen
Adapted heavily from a JPG that was uploaded to Pixabay by Viscious-Speed.
Source Firkin
Formed from a tile based on a drawing from 'Viaggi d'un artista nell'America Meridionale', Guido Boggiani, 1895.
Source Firkin
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
Turn your site into a dragon with this great scale pattern.
Source Alex Parker
If you like it a bit trippy, this wave pattern might be for you.
Source Ian Soper