Ian initiated a new feature for WordPress themes that will be rolled out with version 2.7: Child themes. You can think of it as a cascading theme hierarchy.
For example, say you download a theme (parent theme) and then want to add a custom contact page. Instead of adding that custom page template to the main theme folder, you create a “new” theme (child theme) with only that one custom page and then link it to the parent theme. Viola! A complete, custom theme with the original theme left untouched, or in other words, dead simple to upgrade.
Commentary
You’ll need one more file in that Child Theme: style.css.
Still pretty simple though. :)
Ah, yes, style.css. Thanks!
But isn’t it more difficult to run, compare to if you use a plugin (in this case about contact page)?
What do you mean by more difficult to run?
If you’re talking about speed, I imagine the difference will be negligible. It will be in WordPress’ best interest to make it as fast as possible.
And maybe a contact page is not the best example. I was thinking of a more complex custom page rather than just a plugin.
Like say you have ten custom pages you create and several others from the parent theme that you modify. Then having a child theme starts to make more and more sense.