content touches the edge of the browserwindow possible?

hello,
im trying to design a theme. my issue is that the content (the whole content of the blog incl. header & sidebar) doesnt touch the top and left side of the browser window. there is always some space between.
is it possible to fix that?
here is an example screenshot:

https://ritalinray.de/sample.tiff

the white box on the top is a placeholder for my header, which i want to come aout of the upper-right edge of the window. the navi-sidebar should be where it is now but with the lines go to the left border of the window without stopping.

thanks!