How to set top margin in twenty fourteen theme

I wanted to know how to adjust my top margin in my CSS code. (Or if there is an easier way) So that there is no blank white space (0) on the top of the page HERE in my twenty fourteen WP theme. I am using “my custom CSS” plug-in. Here is the current code: Where and what would I put?

.page-template-template-nosidebars-php #primary{text-align: center;}.site-content .entry-header,
.site-content .entry-content,
.site-content .entry-summary,
.site-content .entry-meta, .page-content {
/* Original max-width: 474px */
max-width: 75%;
margin:0;
}article,
aside,
details,
figcaption,
figure,
footer,
header,
hgroup,
nav,
section {
display: block;
}
header {display: none;
}article,
aside,
details,
figcaption,
figure,
footer,
header,
hgroup,
nav,
section {
display: block;
}
header {display: none;
}