How about make a dark template for this forum?

There's a lot of ready-made templates out there for this forum software so there's no need to build one. But implementing another template in a forum always means a lot of work and testing, I know what I am talking about.
If you like it not so bright (I dont't, too) so you can change it in your profile. Click on the left upper corner on your avatar, choose settings and there you can change the style. There are four different ones, but only Blue Temptation has not such a lot of white backgrounds.

Zitat von falanca

lso it would be better for OMV system login page.

This is already possible:
Theming

Zitat von votdev

Hmmm, all pages should/must have a coorperate design.

Not down to the last detail (But please don't change that, the Blue Temptation style looks IMHO a little more eye-friendly). In this style some coloring is different to the standard style, a slightly better contrast between i.e. the user info area and the posts area.
You can see that easily if you open two forum views in their own tabs with two different styles loaded.