Templates reset to default and designer page is blank dark page (opacity-0)

I can’t save templates! Everytime I update them, they reset to their default settings. Still I tried to customize a product, I get a blank and dark designer page. I inspected using inspector tool, the page’s opacity is automatically set to 0 for some reason (maybe because there’s no template?).

I temporarily disable opacity: 0 -> opacity: 0, the designer page appears but yes without a template… help! Need it to work asap.