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The RegisterLogin div is overlapping the title in the forum module. Also, the "currently online has a left padding of 20px" which makes it look odd.
Make the forum search form all in one line. Right now it's in two lines.
Make the "go to end" link of the topic view in one line. Right now it's in two lines.
The "post count" font size is too big in topic view.
If the sidebar is too long (like when there are too many widgets), the bottom displays a white space.
You also might want to consider adding breadcrumbs.