Replace the flat radial green with a layered body background: a soft
overhead light and an edge vignette over a fine grain tiled across the
green gradient.
The grain is a tiny seamless SVG tile (feTurbulence fractal noise mapped
to semi-transparent black speckles, stitchTiles so it repeats without
seams) committed under web/assets, so it rides the existing asset
pipeline and works offline; the palette and panel/card colors are
unchanged.
