If you use the standard [ code ] (extra spaces added to show what I mean) tags in an entry then I believe that ExpressionEngine places this information into a div with a class of the name codeblock and also inside of <code></code> elements if you wish to style those instead so you should be able to do something like this in your CSS file :
.codeblock {
background-color: #fbfbfd;
border: 1px solid #dae0e4;
font-size: 11px;
margin: 10px 4px;
padding: 8px;
width: 740px;
overflow: auto;
}
Change any of the widths and items above to whatever you need to get it to look right in your design though 
That should work hopefully?
Best wishes,
Mark