If you're getting a template from an external source, you can upload what you get using the restore procedure, too. If you get a template as text, you'll install using the "Edit Template" window. If the blog uses a Classic template, the "Edit Template" window will, of course, be the only choice.
If the blog uses a Layouts template, you'll have the use of the Browse then Upload buttons, which will only upload XML code - so you'll have no chance to make a mistake. If the template comes as a text file, it might be HTML or XML, so make sure when pasting the code into the "Edit Template" window that the blog uses a Classic or Layouts template, respectively.
Installing an HTML template into a blog that uses Layouts, or an XML template into a blog that uses Classic, will cause you various problems, some which you may see immediately, others only later. If you use the "Edit Template" window to install your template, make sure that you have the right type.
- If the blog "Edit HTML" wizard has an Upload button, but the template is HTML, you won't be able to use it until you Revert the blog to use a Classic template. Don't try using the "Edit Template" window, until you Revert the blog.
- If the template is XML, but the blog "Edit HTML" wizard has no Upload button, you will need to Upgrade the blog to use a Layouts template. Don't try to use the "Edit Template" window, to install an XML template. If the blog is published by FTP, you won't have this option at all.
- Use the "Edit Template" window ONLY if there is no "Upload" button, and the template is HTML.
- If the template is XML, ONLY use the "Upload" button.
- If you follow all 4 of the above rules, you'll be fine. Since you're here, you probably ignored one or more.
You cannot fit a square peg into a round hole, nor a round peg into a square hole. Neither can you make a Classic template work in a Layouts blog, nor a Layouts template work in a Classic blog. Before you load a third party template using the "Edit Template" window, make sure that it's the right type of template.
Having said that, I will note that you can use HTML in a layouts template, and that is easier to do than you would think. That's still a layouts template, though - there is no way to fit XML code into a blog that uses a classic HTML template.