Super subtle indeed, a medium gray pattern with tiny dots in a grid.
Source Designova
Inspired by a drawing seen in 'City of Liverpool', James Picton, 1883.
Source Firkin
Prismatic Groovy Concentric Background 4 No Black
Source GDJ
This pack of filters can help you adding a blocky overlay to objects. May come handy at drawing blocks of stone.
Source Lazur URH
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Prismatic Hexagonalist Pattern No Background
Source GDJ
I asked Gjermund if he could make a pattern for us – result!
Source Gjermund Gustavsen
Prismatic Geometric Tessellation Pattern 3 No Background
Source GDJ
A seamless pattern based on a rectangular tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
From a drawing in 'Cassell's Library of English Literature', Henry Morley, 1883.
Source Firkin
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin