1. Open /home/git/go/src/github.com/gogits/gogs/templates/base/head.tmpl

Add after:

	<!-- Stylesheet -->
	<link rel="stylesheet" href="{{AppSubUrl}}/css/semantic-2.2.1.min.css">
	<link rel="stylesheet" href="{{AppSubUrl}}/css/gogs.css?v={{MD5 AppVer}}">

This line:

	<link rel="stylesheet" href="{{AppSubUrl}}/css/custom.css">

2. Copy custom.css to /home/git/go/src/github.com/gogits/gogs/public/css
3. Run service gogs restart