One solution is to make use of the html/template functionality, create all your pages to include the same section like below. I would however leave room to add tags to your head by leaving the in each of your pages.
{{define "page_template"}}
<head>
<title>My page template</title>
{{template "template_css"}}
<!-- page specific css if required -->
<link rel="stylesheet" type="text/css" href="/assets/additional.css" />
</head>
... etc ...
And the template_css:
{{define "template_css"}}
<link rel="stylesheet" type="text/css" href="/assets/allpages.css" />
{{end}}
A snippet of code for the template parsing
tp, err := template.ParseFiles("page_template.html", "template_css.tp")
err = tp.ExecuteTemplate(buf, "page_template", templateParameters)