Deflaut theme - div#columns higher than #header :-0?

[Doberman]

Hello,

As we know deflaut prestashop theme has div#columns which contains left, center, and right columns.
But when we put something like this into global style

#columns { background-color:#cccccc; margin-top: 10px; min-height:20px; border: 1px solid #aaaaaa; }

then we'll see that div#columns is higher than #header. Does anybody know how to fix it?