I write this to inform about what which might be an error in Joomla or Joomlart templates.
For my site through my ftp tool, I have created and edited a custom.css file.
Recently I wanted to add more code to custom.css. Since I did not have my ftp tool available, I choose to edit custom.css from admin / template.
I opened the file. It was blank. I did not give much thought to it, and added the new code. Then, the code I had entered earlier did no longer function. Only the new code. Fortunately, I had a backup of my old custom.css and could re-enter the code which was gone.