Assuming Stormtroopers only wear their armor while on duty, what do they wear off-duty, sleeping, etc (Finn doesn't count)?
First of all, we know Stormtrooper fashion originates from Clone Trooper fashion on Kamino:
The following points are true or probably true about the history of trooper fashion:
- When you're at war, off-duty uniform fashion is the last thing on everyone's minds, especially when national budgets are tight and your financiers are the enemy's banking corporations (for that matter, have any bank in any galaxy in any time period ever loaned people money for fashion?)
- During the rise of the Empire, the focus was on suppressing individuality and promoting monochrome fashion
- During the fall of the Empire and the rise of the First Order, the focus is on revamping the military's combat effectiveness to reconquer the galaxy. Again, off-duty fashion is the last thing on everyone's minds
Therefore, what Finn is wearing underneath the armour he shedded:
is the closest we have to what a First Order Stormtrooper wears off-duty. It probably isn't too far off from what Imperial Stormtroopers wear. They probably couldn't be bothered to do a redesign.