Stockholm is very safe during the day in its popular public spaces and attractions. Safety at night however depends highly on your location. I personally would not recommend being out by yourself late at night, and would advise keeping somewhat sober and keeping an eye on your drinks while out enjoying the night-life as drink-spiking and getting harassed by drunk men is a concern here.

Stockholms pretty safe as long as you don't get yourself into dangerous situations, such as walking alone at night or going to sketchy places. As long as you use common sense you'll have a great time!