To be honest I don't remember either, but that's how I imagined them in my mind. Similar in fact to the Bedouin or other desert people. The frequent references to jihad and living in the desert was what made me picture them this way. Also I imagine a tight leather suit in the desert is likely to get very hot, very fast

Interestingly I just read this on the Wikipedia page for Fremen
"See also:
The Tuareg people of real-life Sahara, who bear many resemblances to Dune's Fremen. In the own language, the Tuareg word for themselves means "Freemen"; this may or may not be a coincidence."