The Fiery Feed 1.3.1 update brings new themes, including a theme for every color the 5C is available in, and a new theme directory. You can find the directory in the app under settings > themes > then tap the + button in the upper right corner.
For those who like to tinker, it is also possible to install custom themes. A theme file is a simple JSON file (see the White Theme as example) defining the colors that should be used in the app as hex strings.
Custom themes can be installed via the URL scheme
fiery://addTheme/{encoded link to the theme file}
As an example, you can open this link on your device to install the sepia theme.