Colored maple leaves scattered on a surface. This is tileable, so it can be used as a background or wallpaper.
Source Eady
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Prismatic Curved Diamond Pattern 8 No Background
Source GDJ
Seamless Dark Grunge Texture. Here's a new grunge texture for use as a background.
Source V. Hartikainen
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Carbon fiber is never out of fashion, so here is one more style for you.
Source Alfred Lee
This background has abstract texture with some similarities to wood.
Source V. Hartikainen
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
A web texture of brown canvas. Will look great, when used in dark web designs.
Source V. Hartikainen
This could be a hippy vintage wallpaper.
Source Tileable Patterns
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin