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First I'll go after the main page. I also agree with the background drawing too much attention of the visitor instead of the content. The links at the bottom of the page near the RSS feed are not clearly visible. The font size is small and on mobile browsers would be incredibly difficult to read without zooming. You shouldn't really hide the copyright information either. How it looks now, you practically have to highlight it to be able to read it.
I'm also going to suggest learning how to rewrite urls.
This is going to make it easier for a search engine to understand what the content will be. Having a parameter named "p" is not going to mean anything significant to a bot.
Now lets move on to the forum, starting with the index page.
I noticed many of your links in the header have a white background that goes over the text when you mouse over them. I would really consider having the "Create Account" link do the same thing for consistency.
I'm going to say to cut the avatars in the last post column. Just having the thread title and last poster is enough. With more images it consumes more of a visitors data usage and more of your bandwidth if the avatar is hosted on your website. In the Recent Topics sidebar I am going to once again say to remove the avatars for the same reasons. While I am on the topic of last post information, what I would do is change the thread title to link to the last post instead of the first post. The last poster should have a link to their profile because then a person can easily access that person's profile. The actual date of the last post I would not have a link at all since many forum softwares clicking the thread title in that column automatically jumps them to the last post. You might look into doing a time ago instead of an actual date. After x number of days just have it say more than a week ago or something. It would probably be best to just write a new function for the date instead of changing the software's default so you can easily flip back if you don't like how it looks. This can make a forum look more active than a person seeing a last post date of a couple months ago.
On the forum stats section you can't read the words. You can only read the numbers somewhat. I'd suggest more contrast in the numbers part and figuring out what to do about that background. Also, instead of putting the value for the stat first, I would actually place what the name of the statistic is first. It gives it more of a title element to a user so it would be easier to associate which thing goes to which stat.
Once again, I am going to suggest cutting avatars in the last post section. It isn't necessary content. The rest seems fine.
The first thing I see is a spot that says X number of replies. I don't really feel that adds anything. If it says 0 or a really low number it might actually make someone less likely to post. They might feel strange or odd.
In the authorbit section of posts, I am going to say just have their Steam name only, not additional images also. To me it also seems odd that the top part above the post lists the poster's username, but then have the information below it on the left side. Either everything about the author on top or everything about the author on the left. I personally prefer poster information on the left so the top part you can put the subject for the post. Usernames should be clickable to take you to the member's profile.
You also will want to look into caching images and other static content to boost your Page Speed ranking in Google. You also seem to have a lot of blank lines in your HTML. Try and reduce this because it slows down the rendering of the page.
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