Abstract Arbitrary Geometric Background derived from an image on Pixabay.
Source GDJ
A seamless texture of a rough concrete surface.
Source V. Hartikainen
Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Brushed aluminum, in a bright gray version. Lovely 2X as well.
Source Andre Schouten
A seamless design of flowers remixed from a jpg on Pixabay by Prawny.
Source Firkin