In computer side, a thing (either the window manager, a specific application, or some other user-visible thing) is themable if the way it looks can be changed, usually on the fly.
For example, in Windows (since 95 Plus! pack), a desktop theme contains a background image, icons for things like the My Computer and other system things, sounds for system events, pointers and a screen saver. The word "theme" describes the fact that the elements all have same theme; For example, the "Mystery" theme that came with Win95Plus took all elements from cheesy detective stories.
In X11, many window mangers support theming, either indirectly (you can change the look, but dynamic changing isn't supported - needs hacks the user needs to do) or directly (specific menu to change the theme, or even has a direct support for theme building and changing). Examples of window managers with excellent theme support include Window Maker and Enlightenment.
Some web sites are also themed, usually this just means that the user can change its colors and some other things of its outlook and save that to his/her specific user profile... Examples of themable websites would include themes.org (naturally) and, of course, Everything2.
See also: skin