You use mainly green (dark, light, with little of yellow), grey and brown (beige, with little of grey) in the new design. All of them are mixed in a way to match each other with the help of their shades and tones. And now take a look at forum supertitles - they are turquoise. It's blue color mixed with light green. You don't use blue color anywhere else.
what should I preferably do to making them match?
I think you have some main colors (I highlighted three and mentioned them before) and in your new design you need to use only them in different tones and mixing them with others. So you need to use green with adding some blue in supertitles (still not good in my view because you don't use blue anywhere else, but at least it will be better) or you can add blue tones in some other places.
It's about colors matching each other. But the problem with this color is also that it's bright while other colors are mostly muted and mixed with grey. I looked at this color in different angles and every time it's different. So I think it may look differently on different PCs. And all the time I looked at main forum's page I always concentrated my eyes on supertitles, but I need to concentrate on Recent topics.
And finally, Paul, as far as I know you don't like stereotypes and like to break these "rules". But you need to remember that may be all of these is just stereotype, but you are developing MiniBB for people that have these stereotypes
, not just for you. Some long time ago when I used MiniBB for my forums I designed it to absolutely match color scheme of my site (at this time it was dark blue). After this I had a lot of complaints that I changed the backcolor and font color of posts from black on white to dark blue on light blue. They said that it was inconvenient for their eyes. So I was to change posts colors to black on white again while it didn't match anything. But I knew that I don't want these people just go away from my forum or use it less just because many of them don't like what I like.
So what about MiniBB I think you should create design that people can install and don't have complaints about it from their users. Or then they will be thinking how to change these colors (and most of them don't know it I guess) and coming to Minibb.com with a lot of the same questions how to change them. And you can use color scheme that you like
at Minibb.com or Demo site not at standard package.