You cannot build or use your own custom theme built from scratch at WordPress.com.
http://en.support.wordpress.com/ftp-access/
http://en.support.wordpress.com/themes/adding-new-themes/

The only themes we can use are those found here > Appearance > Themes
This is a multiuser blogging platform. All blogs wearing the same theme are using the same underlying template and we cannot access the underlying files and edit templates or themes. http://en.support.wordpress.com/themes/editing-themes/

There are two free themes, Sandbox and Toolbox designed to allow maximum flexibility for CSS editing and creating new “skins”. Toolbox is more modern and includes a lot of support for CSS3 and HTML5, so it is really only limited by your skills and knowledge.

If you have CSS editing experience the upgrade will allow you to stylize the appearance of themes you find here > Appearance > Themes, but will not allow you to change the functionality, by editing the underlying template. It does not allow you to remove footer links to the theme designer or to WordPress.com. Also note that CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes at WordPress.COM. http://en.support.wordpress.com/custom-design/custom-css/

I am currently trying to figure out how to use WordPress as a CMS, I'm a web designer and write code by hand for both HTML and CSS, I've figure out how to code my own theme. However, I am now looking to build an image gallery that my client can then update themselves. I can build the gallery the way I want it fine on my end, but it won't be changeable to a layman. So I figured what I should do is find a plugin that displayed images the way I want, but so far they are all too fancy:( Its probably not possible for me to just code the right look and then find a plugin that'll work with it right? Here is a screenshot of what I'm trying to do. Can anyone help?

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