How to install template

I have made a copy of the theme folder via FTP and changed the XML and folder name. It installed ok, but when I want to activate the theme I get this error: "0 JForm::getInstance could not load file".
What must I do? How can I create new theme and upload it? What am I doing wrong?
Thank you

Hi Davor,
Why did you use the function "Copy theme" at backend > OS Property > Manage OSP themes.
And then, you will be able to modify the copied theme.
Or the better solution, you can backup the original theme and then, you can modify it without any problem.
Thanks
Dam

That is what I did for now. But am still curious how to do it manually and then install it. What am I missing?
Thanks

Hi Davor,
Please submit ticket and send your site information with super admin account. I will check and explain problem for you
Thanks
Dam