How much style changes before calling a theme your own?

I have a courtesy question about themes.

I downloaded a basic clean white theme. There is nothing fancy to this theme. It looks simpler than the default theme. While I am very new to WordPress I am quite familiar with XHTML and CSS. I set up my localhost server system on my Mac using MAMP so I test my theme locally.

I have gone into the CSS and changed the code quite considerably. I have a new banner image, added a fixed background page image (for visual effect), created my own top level navigation (points to other static pages), changed the content width, changed the styles for all the posts & sidebar links, changed the footer (with graphics), changed all the margins & padding, changed all the font sizes from pixels to my own custom EM’s, changed the fonts, changed roll over colors, etc, etc.

So, while some folks do a little tweaking here and there I have gone and changed just about every line of CSS. Some of these changes make for a totally different viewing experience. So, my question (finally) is do I need to still include text on my site that says “theme developed by John Smith” ?

AFAIK, most themes are developed by adding styles to the PHP and XHTML framework of the WordPress install. When modifying a theme is there a line in the sand that allows one to call the theme one’s own and not recognize the beginning theme’s author? This must be a gray area. I did not create the PHP and XHTML from scratch nor did I build the CSS one line at a time. But I have modified my CSS from the original CSS that the pages no longer resemble each other (although most two column WordPress sites do have features and presentation in common.) This theme will be used just be me and will not be a public release etc. It would be nice not to have to credit someone else’s CSS when I have put so much work in building my own “look”.

Any thoughts on the convention and etiquette here?

Thanks.

Photomaximum