HELP! Purple line cannot be found.

Due to this, I am not being able to center my items.

As the others have pointed out, you're being confused by the teacher's line of text being purple and yours being blue. That's purely cosmetic. Just the color scheme of your code editor. Has no effect on your web page.

It certainly has nothing to do with why your code is not centering your items. We'd need to see your entire page (HTML and CSS) to tell you what's wrong there.

If you are talking about the line of code being blue instead of purple (on a dark background instead of light), you could switch to the Light code theme in DW's preferences to get your code view to look more like the video. However that coloration really doesn't mean anything in and of itself, other than showing you where a property is when compared to its value.

That's changed under Edit > Preferences > Interface > App Theme (choose the lightest color chip)

Could it also be that, 'the purple line' you are refering to, is the background color of the line in code view?

If so, then the purple color is nothing more than an indicator of the css/html line that is currently selected.

There is nothing in that code that would create a purple line.

Without seeing the remainder of the code, and the tutorial being used, it's going to be pretty difficult to come up with any reasoning for not seeing what happens in the video on your version of the page.