The current default style on the forums is the Minty theme. Many things could be better in there. And the other hidden choice, the "prosilver-minty", is not much better. And most people are not even aware this second choice is available. But it is just the same as the default Minty anyway, the same but with different colors. There is a need for further improvements.
Those dark green tints are nothing close to mint green. That's some kind of khaki green, I guess.
You have this nice but somewhat distracting vibrant yellow-green on the body background, i.e. on the margins. It brings the user focus on the outsides, especially when trying to read those darkened posts with their greyish green backgrounds.
The posts alternates between white and some kind of dark off white backgrounds. Those greyish green backgrounds. Thus you get to read some well contrasted and luminous post, made of dark grey text on white backgrounds, and then the next post is noticeably much darker, dropping into a quite lower contrast. Of course, this helps in separating the posts from each other, but there are other ways to achieve this.
There are two kinds of hyperlinks. The ones that are part of the forums themselves, like for going from a topics list to a chosen topic, and the other ones: the links posted within the user's posts. The first ones, the forum's links are indistinguishable, getting approximately the same color as for normal text: #444C3C vs #333333. So, the user must guess where the links are ! And the other kind of links, the ones inside posts, well they are very light, they clearly lack sufficient contrasts, especially on visited links. To sum up, all the links looks very bad.
The "Linux Mint Forums" title, standing next to the logo, is just too small. And the "Welcome to the Linux Mint forums!" under it is just so small, I tend to ignore it. Thus, I mostly just see the logo and the title in there, and this title then feels like having a very bad vertical alignment. And also, the font is totally "generic".
The page header is made of two separate elements: the header bar with the logo and the navigation bar under it. This feels too modular. Every element is a separate box with rounded corners. If at least the logo, search bar and navigation bar were grouped together into a single rounded box, that would already feel better, less modular.
That's a bit hard to explain. Usually, on your "standard" Minty theme, the content text is made of dark gray: #333333. But when you click on a link to access a specific post within a topic, then the text is made of pure black: #000000. Take this Forum and Topic Icon Legend for example. Here, you click on that link to the topic, and you get normal text, in dark gray: #333333. But now, when you try to get to the same place, but using a link to the first post of that same topic, you get a pure black text: #000000. Test it: Forum and Topic Icon Legend. You end up in the same place, but with a different text color. That's bad. Most modern websites are not using pure black text on pure white backgrounds. It is all explained there: Why You Should Never Use Pure Black for Text or Backgrounds.
The quotes are just another kind of boxes, again. And when facing quotes inside quotes, you are just facing hardly distinguishable grey boxes inside grey boxes.
Some titles are hardly distinguishable, being just slightly bigger bold text, with no noticeable color difference.
And lets not talk about those annoying ads that got added recently. I don't care much, because the design is already bad. But if the forums had a good design, those ads would just be in the way.
And there are still more things I could talk about but I will stop there... Images are worth a thousand words... Just take a look ! Everything got refreshed, and yet the coding is kept small and professional.