modificar [code]

[ilusoignorante]

sustituye solo el borde, osea, que quede asi:

.codeblock {
background: #fff;

border-top-style: solid;
border-top-color: black;
border-top-width: 1px;
border-bottom-style: solid;
border-bottom-color: white;
border-bottom-width: 1px;
border-left-style: solid;
border-left-color: black;
border-left-width: 1px;
border-right-style: solid;
border-right-color: white;
border-right-width: 1px;

padding: 4px;
}