There are fairly vague terms. "Low key" means that the majority of the frame has areas that are underexposed (dark) with small areas at full exposure (sometimes even somewhat hot and overexposed.) This tends to also mean that the image is high contrast, which is confusing (low key = high contrast).
"High key" means that the majority of the frame is at full exposure with very few areas of darkness, so high key = low contrast. A brightly-lit musical number from a 1950's Technicolor musical like "Singin' in the Rain" might be considered high key (although the actual "Singin' in the Rain" dance number is somewhat low key...)
A good example of a low-key movie is "The Godfather, Part II" or many film noirs.
Now "dark" is a somewhat subjective word and sometimes it can mean a low-key image with lots of black shadows, so is high contrast, but it also be a low contrast scene that is flat but very underexposed overall, so as to create the feeling the low-level murkiness with a lot of ambient detail. "Panic Room" is lit like that, sort of "dim" more than "shadowy".
Kickers are a matter of taste and style -- they can be unmotivated if that works stylistically for you, otherwise you should be selective just as with backlights. I tend to only use backlights when motivated like by sunlight coming through a window, so when I use backlights, they are often nuclear and super-hot, and motivated. Otherwise I avoid them in day interiors if coming from an illogical angle because then it makes the scene look "lit". For night interiors and exteriors, it's easier to make up a light source that could be causing a backlight or kicker.
I do use kickers a lot, but soft ones, more to create a soft sheen off the cheek or something -- this is less distracting and looks more natural, as if caused by an off-camera window or lamp or a bounce off of a wall. Sometime you can add a soft kicker just by putting a white card behind someone's head to kick some natural light back, or a silver card. Or you can use a soft light -- I like to use Kinoflos for soft edge or kicker lights. If you watch the movie "Crimson Tide" you'll see a lot of soft Kinoflo edge lighting on people.
