It is not css-related issue.
look in skin1/single/home.tpl for
Code:
{ include file="rectangle_top.tpl" }
If there is width specified:
Code:
{ include file="rectangle_top.tpl" width="800"}
delete it.
If there is no width
Look in your skin1/rectangle_top.tpl for
Code:
<table class="Container" cellpadding="0" cellspacing="0" width="{800|default:"100%"}">
and change 800px to 100% or place original code there
Code:
<table class="Container" cellpadding="0" cellspacing="0" width="{$width|default:"100%"}">