Beocat

Yes, this function does exist. However if you are running on a mobile phone or using a mobile browser, you may not be able to see it. First you will need to change your browser to a desk top site browser. This is usually in the settings area of your browser if you are on a mobile phone. After that you should be able to see the option to mark the entire site as read or to jump to unread content in the bottom right side of the browser. Hope this helps!
 

 

Click under the post where it says "Updated 11 minutes ago" for example and click on that link. It should take you directly to new content, though you may have to scroll up to get to the oldest of the new content (depending on how far behind in the thread you are). It may not show the "new content" line on threads you have not visited in several weeks that have recently become active again. 

So by dark...do you mean tragic? I'm trying to piece this together to help you but dark means so many different things to different people. 
 
I'd recommend Bacanno! It's a fairly dark comedy, really a masterpiece even. Has some romance in it, some gore (but not in excess as some shows do), some psychological terror, suspense, mystery, comedy...really it has a lot to offer. The story itself moves extremely fast however, so you need to give it your full attention if you watch it. Otherwise you'll be lost after just one episode.  Romance is not the main focus here. The comedy is. It is still there intertwined in the story.
 
Norn9 is also a tragic romance you may want to look into. It has dark elements to it, some mystery and suspense as well. Beautifully animated. 
 
Either of those two pique your interest?
 
Hmmm...there is always Future Diary as well. That one is very special. You can't forget it.