I would think so with the right CSS adjustments. If you want one of us to look we will need a link however.

Currently using default theme : http://www.warlegend.net

We would like to achieve something similar to this with the background (really temp mockup but at least you get the idea)

http://dhainne.free.fr/forum-mockup.jpg

That could be done by using some transparent backgrounds for the Simple:Press theme so your main background is displayed under the forums.

It would require a custom theme and special CSS. You would first create a custom SP theme and then edit it from there. If you want to create your own theme check out the info in the codex Creating Custom Themes.

There is also a paid custom theme service if interested. https://simple-press.com/custom-simplepress-themes-for-every-need/