Alright well in Entertainment Discussions we have the following:
General Entertainment: This is basically the section where you go to talk about the media, such as TV shows, movies, books, celebrities and so on. Fairly straight-forward. While I'm going to leave it up to you to see who the good posters are (it's subjective anyway), some of the more active individuals here would be Mr Cat Dog (who previously modded it after myself), Harlequin, Shining Raichu, TRIFORCE89, and Scarf. They would probably be the most notable and active members of the section.
Video Games: This one is fairly straight-forward. It's simply the section to discuss video gaming, with sub-forums for Online Gaming and another for LP's, Guides (for games) and reviews on games. The main forum itself houses just discussion on video games. Some of the notable members there would be Captain Fabio (who previously modded it), Klippy (probably not so active but also used to mod the section), I usually enjoy TRIFORCE89's posts, and from there, I don't know of too many active members, the section kind of lacks them in that department. I go there myself from time to time.
Technology and the Internet: Pretty nice section for talking about technology such as computers, phones, gadgets and that sort of thing. As well as discussing Internet fandoms, trends and so on. Hybrid Trainer, Team Fail, Gerri-Shin, Zet, twocows, they all tend to make quite good posts there. Not sure on current activity though.
The last two I honestly don't go to very often (if at all), Japanese Entertainment and Tabletop Games are both moderated by digi-kun. They're pretty self-explanatory based on what I said. I just noticed that the latter section is pretty dead, but JE still gets a decent amount of posts if you like Anime and so on. If you're interested in that culture then it'd be a pretty good place to go. However as I say, I don't go there myself.
As for the "theme" business, I'm assuming it's referring to when people choose an avatar/signature combo based on a particular character, show, movie, whatever really. They may also change their usertitle to suit. Currently you could say I have a theme, but I just like the character a lot so idc. It's nothing important, just a "term" people use. A lot of members here put effort into their themes as they may be graphic enthusiasts, or interested in design.