Loaded JSON not center..

[prabhathAmila]

hi
I create design and download it as json file. (chrome)
And I upload it in to fabficjs again. it not centered properly(firefox)
how can I center all platforms.
ex.
https://www.shutterstock.com/editor/
create template and save in chrome. It will open firefox. it always centered. how can I do that?
Please help