I want to host a blog with funny and dark articles. How much do I have to worry about the appearance. Should I focus only on the content or does the visual appearance and graphics matter a lot too?
It depends on the audience you're trying to attract, but on my experience, the visuals count a lot for a modern blog in terms of attracting readers. You don't need them to be flashy, but they need to look polished, and to match the content.
One good way to proceed that isn't too difficult is to go with a very minimal design, but make sure each article includes an interesting image. (You can check the link in my profile for an example of this approach.)
Of course, all the visuals in the world won't overcome weak writing (unless the blog is primarily visual focused).
The short answer is "a lot".
Anything you publish - whether it is free or paid-for content - has to compete with an awful lot of other websites, blogs, books and other sources of information.
When someone clicks through to your site, you have about two tenths of a second to capture and hold their attention long enough for them to want to continue reading.
That is not true quite so much if what you are writing is technical because someone will stick with you if they actually need the information you are offering but, in the world of entertainment, you only have that two tenths of a second.
Here's a test you can do for yourself: start surfing the internet for material of the same sort you intend to write. Look specifically for people you have never heard of before and reflect on your own responses to poorly designed websites. Even better, given that you will have a certain in-built bias now you have posed the question, get some friends and family members to rate a list of websites you give them. Just let them do their own thing without any prompts or direction from you other than "do you want to read this website?".
See http://www.entrepreneur.com/article/223150 for more on the subject.