
Welcome Kalagaraz!
Hhmmm....I can't think of any really good way to do this.....
You could draw a simple circle or rectangle that is larger than the polar grid. Give it some color, any color (you could even use a gradient). Place it behind the grid, and make sure it completely encloses the grid on all sides. Duplicate the grid, then ungroup them, then do Path menu > Stroke to Path, to both of them. The deselect both and select one of them, and then Path menu > Combine. Then select those paths and the background object, and do Path menu > Difference. Then you'd have to do a bunch of node editing, to correct the minor, yet important distortion caused by the Stroke to Path command, by snapping nodes of the cells to the 2nd and still whole grid. A LOT of work!
Maybe someone else will have a better idea
