My first impression from looking at your seat is that there was a hot container(round) full of hot liquid (food?) sitting on seat which spilled some or all of its contents. Probably happened on a holiday, someone traveling to family function - and got caught up in some spirited driving...OOPs!
To correct to perfection = new original cover/new passenger seat installation $$$
To correct to minimum standards of acceptance = find and install decent seat cover - best match(maybe you can even find and install bottom half only?)$
OR...(on a lighter note)use this seat for spill testing when you do street trials for your new "spill proof passenger seat take-hot-soup-to-party contraption"