I see what @ezio means about the apparent overflow when the bodywrapper left margin is at 20%. (It's not really overflow, since the sidebar is actually the width of the screen.) How about this change to basic.css?

 44 div.sphinxsidebar {
 45     float: left;
 46     width: 20%; /* was 230px */
 47     margin-left: -100%;
 48     font-size: 90%;
 49 }

It looks pretty good to me. The only thing that overflows is the search-input box. CSS for that is set in both basic.css and default.css (which takes precedence) as:

104 div.sphinxsidebar input {
105     border: 1px solid #98dbcc;
106     font-family: sans-serif;
107     font-size: 1em;
108     max-width: 100%;
109 }

I added line 108 (max-width). And now everything looks decent when zoomed in.