index.css

#toolbar1 select {
    font-size: 10px;
}

html, body {
    height: 100%;
}

body form {
    height: 90%;
}

a:hover {
    text-decoration: underline;
}
a:active, a:hover {
    outline-width: 0;
}
a {
    color: #0366d6;
    text-decoration: none;
}
a {
    background-color: transparent;
}
* {
    box-sizing: border-box;
}

[live-editor] h1, [live-editor] h2 {
    border-bottom: 1px solid #eaecef;
    padding-bottom: .3em;
}
[live-editor] h1 {
    font-size: 2em;
}
[live-editor] h2 {
    font-size: 1.5em;
}
[live-editor] h3 {
    font-size: 1.25em;
}
[live-editor] h4 {
    font-size: 1em;
}
[live-editor] h5 {
    font-size: .875em;
}
[live-editor] h6 {
    color: #6a737d;
    font-size: .85em;
}
[live-editor] h1, [live-editor] h2, [live-editor] h3, [live-editor] h4, [live-editor] h5, [live-editor] h6 {
    font-weight: 600;
    line-height: 1.25;
    margin-bottom: 16px;
    margin-top: 24px;
}

[live-editor] {
    font-family: -apple-system,BlinkMacSystemFont,Segoe UI,Helvetica,Arial,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol;
    font-size: 16px;
    line-height: 1.5;
    word-wrap: break-word;
}

[live-editor] blockquote {
    border-left: .25em solid #dfe2e5;
    color: #6a737d;
    padding: 0 1em;
}

[live-editor] blockquote, [live-editor] details, [live-editor] dl, [live-editor] ol, [live-editor] p, [live-editor] pre, [live-editor] table, [live-editor] ul {
    margin-bottom: 16px;
    margin-top: 0;
}

/* Override default styles of source-preview here: */
/*[live-editor] > .editable-source {
    white-space: pre-wrap;
    word-wrap: break-word;
    tab-size: 4;
}*/