From now on you can also set a theme for each of your Categories as well. With this come a few changes in way the theme settings are applied to albums.
Until now, when you created an album, its theme was determined by the "theme" setting from the Control Panel's "Album Layout" tab. The new system applies the theme from an album's parent category to the album. You can still choose a different theme for each album, but an album on your homepage will by default have the theme set for the homepage. The same applies for your categories : an album within a category will have the default theme always equal to the theme set for the category.
To sum it up : whenever you change the theme on your homepage or a category, the albums within which do not have a theme selected will be automatically displayed with the new theme.