To avoid unwanted changes in you blog design.you have to get backup so that you can restore it later.you can save the file in your computer that can be upload to restore your template.
How to get backup of template
It is very easy to get backup.First login to your blogger accountNow click on the left side templateAt the top Right corner you can see backup and restore option as you can see in image above.click it.New pop up appear to get back up clickdownload full template
Now you template file is downloaded in your computer.keep it save.