Thanks Wallace and Nelo. I am so happy with the dark style..you have no idea how happy I am (well maybe you do

)
for phpbb forum, you just download a style of your choice from phpbb.com (just google for phpbb styles). And then in your Admin Control Panel, you just choose to install it, it'll detect what styles you've uploaded if it's not already installed.
Lucid Lime stood out to me... but it required that your phpbb forum is the same version as the extension that's it's designed for. Lucid Lime's version was only available in phpbb 3.2.2 which is later than my version of phpbb which is 3.2.0. So it wouldn't let me install. But earlier versions of Lucid Lime was for much earlier version of phpbb 3.2.0. Which was bugging when I tried to install it.
So I guess 3.2.2 can't be that much different from 3.2.0 so I tried setting version in the style.cfg file to say 3.2.0 instead of 3.2.2 and then my phpbb would let me install it. After trying all the regular browsing on the forum with the change. I noticed nothing is broken so that's it.
I commented on phpbb.com that's what I changed to allow me to use a style that was designed for 3.2.2 to work on my 3.2.0 forum. Someone said I might need to change more than that but from my tests nothing is broken....
What's great is users can choose their own preferred style from User Control Panel. So if something is broken (I doubt it) later, I can always change it back..nothing is lost.
UPDATE: Wallace, oh you do mean for users to choose Dark mode? just go to User Control Panel > Board Preferences tab > My Board Style > Choose Lucid Lime.
Lucid Lime is the dark mode. Prosilver is the usual bright mode. Those are the only you need to be concerned with nd other styles are buggy or not useful, I just never deleted them.