DESTDIR is a feature which lets you install into a tempdir. The idea is that you install it all there and then atomically replace the ORIGINAL version at the non-DESTDIR location.
It's very bad to set DESTDIR to out because the outdir is used after installation, too (your program will stay at that place). This is not expected by DESTDIR. So for example let's say you'd have DESTDIR=/tmp PREFIX=/usr . Then for example a theme is first installed to /tmp/usr/share/themes (by make) - but after it's all done, the distribution is supposed to move it to /usr/share/themes atomically. The program will *always* (also when its still installed to /tmp/usr/share/themes) try to load its files from /usr/share/themes.