There are just two points to be noted for a change in the template of a blog.
- Backup the HTML template.
- Backup the CSS settings in “Template” → “Advanced” → “Add CSS”.
Maybe what you need is just changing some properties of a CSS class.
For instance, you’ve set the body background colour to be magenta and
you want to change it to orange. This can be easily done if you put an
external CSS style sheet to somewhere public on the web and include a
link to the CSS style sheet in the HTML template of your blog.
very short documentation on Google Drive explains how the hosting of
one’s custom CSS stylesheet(s) on Google Drive can be done.
(The link is now dead.) Apart from shortening the time taken for
changing the appearance of a blog, it avoids the loss of CSS code due
to Blogger’s problems and/or users’ mistakes.