NEVER set a new theme as the "forced default" until you have tested it!
You have probably made a mistake naming the folder you put it in. You MUST use the correct name, and case is important.
(i.e. the folder name must match the name of the .cfg and .css files inside the folder. e.g. "subSilver" won't work if you put it into a folder called "subsilver" or "subSilver2")
If you can't fix it, then this script will let you get back to the default theme: Knowledge Base - Set subSilver as the default template