WordPress.com blogs are hosted for no charge. You cannot add or modify any PHP on WordPress.com. If you could, we would have no security. Javascript and CSS editing are also restricted for security reasons.

You can select from a wide assortment of themes for no charge. Some of these themes allow you to customize colors or images while the rest of the style is fixed.

CSS is a technology that adds style to HTML. A CSS stylesheet contains rules that tell a browser how to display a page. Every theme has a stylesheet and you can not edit these on WordPress.com.

CSS stylesheet rules can be modified by rules appearing later in the code. Example:

blockquote { color: red; }<br />
blockquote { color: blue; }

Because the blue rule comes after the red rule, browsers will color blockquotes blue. The Custom Design upgrade works by allowing you to add CSS rules after the theme's stylesheet.

HTML cannot be modified by Custom Design. PHP cannot be modified. Existing CSS cannot be edited but rules saved in the CSS Editor can override existing rules.

If you require your own HTML or PHP, including themes and plugins not available on WordPress.com, you must install software from WordPress.org on a web server.

Blogs outside of WordPress.com are not supported in this forum. WordPress.org has its own support forum for blogs hosted elsewhere.