Sticky header shortcode not working

[alextokyoesque]

I've been trying to make the header for the website www.tokyoesque.com freeze in place so that it is still visible and clickable when scrolling down, but the code I have tried has shifted some text on the homepage to an undesirable position. Can anyone help me find a way to get the sticky header to work without messing up the position of other elements of the page?
The code I have tried is as so:
/Sticky header/
header#masthead {
position: fixed;
width: 100%;
z-index: 100;
}