If there's a file named background-tile.png in the theme then it will get used as the background behind the other content in place of the background gradient.
73695b0 two-step: add support for tiled background image
src/plugins/splash/two-step/plugin.c | 45 ++++++++++++++++++++++++++++++++++
1 file changed, 45 insertions(+)
Upstream: cgit.freedesktop.org