Minor CSS changes for code blocks

I made some minor changes to the default stylesheet to have blocks of code no longer overflow the edge of the page.

Changes were based on the stylesheet I found at

I added in the following:

pre {
        border: 1px solid #333;
        padding: 6px;
        overflow: auto;
        margin: 0px 1%;
        width: 95%;

Thanks to the Snippets public code repository.

Changes and suggestions welcome!