Everybody has their own opinion on visuals; it's very subjective. But personally, I do believe that simpler is better.
You don't have to make every theme white, you're entitled to however you want your site to look since you're the owner after all. There are a plethora of successful dark themed sites, such as (convieniently) Forum Promotion! It's just the manner in which you blend everything together.
What I think the problem is for the dark themed site (MathLesson), is that the background is not aesthetically pleasing to look at, especially because it is included behind the posts. It's grainy and is a peculiar gradient color, and it doesn't blend well with the rest of the purple-y theme on the header and footer. My suggestion would be to make it smooth and have it compliment the other colors.
I agree that simple is better but simplicity is relative. When it comes to webpage design, the first element in question is the color. Some want it plain and some want it bright and fancy. For me, a simple design is a light background and a dark foreground. Just like paper and ink, the combination is what I always prefer.
It's tough to say. People on LazyForums.xyz were saying the theme was great with no complaints. Myself, I think it looks good, but maybe the small area around posts needs to be white - sort of like what we see on ForumPromotion.
Note, my math forum doesn't have that look now, but the following link on someone else's forum does:
Dark themes are actually much more diffuclt to design and in a lot of cases look pretty bland. Yours is one of the better ones imo. I wouldn't consider FP to be a dark theme. It's a light theme with a dark background.
You can't please everyone. One solution to the problem is to alter the skin so that you have a version with light posting boxes as you mentioned and set it as a second skin.
Some people on our forum hate the skin, but the thing is, most people love it. We actually have a second skin, a plain dark one, that those ones can use and they love that. Best way to please the lot I think ^^
