Digikam is a great tool to manage and work on photos — my personal favorite for some years already. But it’s made for KDE and uses a lot of KDE-related libs and stuff, therefore, installing it in Gnome, Xfce, Lxde and the like will cost about 1gb because of the depenedencies.
Nevertheless, worth it. But it comes with a bright default skin rendering it completely unusable for working seriously on light and color settings of photos.
To add themes, you will have to install an extra package (as decribed here) like this:
sudo apt-get install kde-workspace-data
Yes, this will add more data to your disk, but only 10mb this time. 😉