screen.css is in a subfolder /css/screen.css and might not be editable from the dashboard.
it is not quite clear from your question if you want to move the content towards the sidebar, or if you want to make the content wider so it closes the gap to the sidebar(?)
in the second case, there is also the width of
.postwrap in style.css which you could change:
padding: 20px 15px 0px 10px;
word-wrap: break-word; /* fix for long text breaking sidebar float in IE */
overflow: hidden; /* fix for long non-text content breaking IE sidebar float */
try and change 400px to 600px or similar.