Thank you Benzin, I tried enabling High Contrast, and editing the backgroud .tga files, makes it better but the box was still there.
I fixed it by copying entirely the "Large JonZ Rec Manual and JIRM Inside Pages" settings from the menu_1024_768.ini of the Anhnenerbe Final (which was working fine for me) and replacing the one inside ARB's menu ini file, then enabled ARB again, it works perfectly
