"Page not working" when changing header.php

When changing the file includes/languages/english/header.php to read:

define('HEADER_SALES_TEXT', 'Just when you thought it couldn't get any better!');

and I refresh the store front I get a "Page not working".
I realized that I had to delete the single quote in "couldn't," and make it two words:

define('HEADER_SALES_TEXT', 'Just when you thought it could not get any better!');

In order for the page to load properly.

Is there another way of writing this code?
I do want the tagline to read:
"Just when you thought it couldn't get any better!"

Also.....

How do I change the font size and make it italic?

Two different ways to do that

define('HEADER_SALES_TEXT', 'Just when you thought it couldn\'t get any better!');

or

define('HEADER_SALES_TEXT', "Just when you thought it couldn't get any better!");

There's a number of ways to adjust characteristics about the display of the text. Can use html to identify css type properties such as to italicize text can surround it with <i></i> tags to get a result similar to <i>Italicize This Please</i> or can use css on a per page/load type basis to modify the look of an object found using appropriate tag information.

There's plenty of information on the Internet about how to generally use css to do such things. If having difficulty identifying where in ZC to make a particular change, please provide a sample page so that whomever is reviewing the request can provide specific information.