To add to the above (can't find out how to edit my post):

I think the standard style class for sideboxes is 'sideBoxContent'.
So the new line as mentioned should be changed into:

$content .= '<div id="' . $boxname . '" class="sideBoxContent">';

This also worked for my template and I suspect it will be a general solution for all.

Please let me know the results, if ok I will incorporate this in the next beta.