Add Opaque background color to drawing

[LuxxxLucy]

Per #1 the figure should have a opaque white background so that in the dark mode SVG rendering, black text will still be visible.

PS I do not know how the allgrids10.svg is generated so we will probably need to re-generate it and replace the current one.

also, I believe there is a typo in the README code block, grid should be task? (in the immediate scope there is no variable of grid.