when i go into the "header.html" file of this theme, and try to modify one or all the four links "Home", "Downloads", Your Account" and "Forum" (which are normally located at the top of the screen), all what was written in the middle of the page goes to the bottom of the screen, and what was located at the right of the screen goes at the bottom left, below the rest.

as a conclusion, it's impossible for me to touch these links without creating this king of problem. is there a solution i don't know?

thank you for your help