Yesterday, I saw a Blog Update at admin and did the updating. The result: my modified style sheet was replaced by the original Spring style sheet. And the look of my site became that of the original Spring theme.
I had a copy of my customized Spring theme kept in another folder. I copied it to ox-theme>spring but there was no change.
At Joomla you can use a modified template as another template. You copy theme-1 to theme-2, modify it, and change the Name in the XML file to theme-2. It will be regarded as a different template. I tried it with the spring theme, but it doesn't work.