Example `styles/website.css` styles that over-ride Gitbook's own styles, specifically for code blocks and image alingment (centred).

website.css

/* @import url(https://fonts.googleapis.com/css?family=Raleway); */
@import url(https://fonts.googleapis.com/css?family=Ubuntu);
@import url(https://fonts.googleapis.com/css?family=Ubuntu+Mono);

/* Use Ubuntu font instead of the default Railway font */
body {
    font-family: Ubuntu;
    margin: 2rem;
    font-size: 2rem;
}

/* Show keybindings to look like keys on a keyboard */
kbd {
    display: inline-block;
    border: 1px solid #ccc;
    border-radius: 4px;
    padding: 0.1em 0.5em;
    margin: 0 0.2em;
    box-shadow: 0 1px 0px rgba(0, 0, 0, 0.2), 0 0 0 2px #fff inset;
    color: #000000;
    background-color: #f7f7f7;
}

/* Over-ride maximum page width - default 800px
 - if over-ridden then paging arrows can appear over text*/

.page-inner {
    max-width: 1000px;
}


.book-body p , .book-body h1,.book-body h2,.book-body h3,.book-body h4,.book-body h5,.book-body h6,.book-body div,.book-body span ,  .summary , .summary li , .summary li a  {
    font-family : "Ubuntu", "Open Sans", "Clear Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
}

.book .book-body .page-wrapper .page-inner section.normal code {
    font-family: "Ubuntu Mono", Consolas, Menlo, Courier, monospace;
}

img {
    display: block;
    margin-left: auto;
    margin-right: auto;
}

/* Place thin green boarder around code blocks & increase font size from 85% to 108% */
.book .book-body .page-wrapper .page-inner section.normal .highlight pre, .book .book-body .page-wrapper .page-inner section.normal pre {
    background-color: #f7f7f7;
    border: 5px solid #D9EDF7;
    border-radius: 3px;
    font-size: 108%;
    line-height: 1.45;
    overflow: auto;
    padding: 16px;
}

/* red text for all non-language specific code sections & increased font size to 108% */
.book .book-body .page-wrapper .page-inner section.normal code {
    background-color: #f7f7f7;
    color: green;
    border-radius: 3px;
    font-size: 108%;
    margin: 0;
    padding: 0.2em 0;
}