I checked your admin in detail and found the problem.
Bigcart theme option panel is working well now. Only CSS fields are not working.
Because it is caused by server cache problem. In theme folder, there is already in compiled css code.
But your website is including old css version.
You didn't set up any cache plugin, but your hosting server is provide server cache.
So please contact your hosting provider and disable server cache.
If you have the other question, please let me know
Kind Regards, Amand