discourse/app/views/common/_discourse_stylesheet.html.erb

    <%- unless SiteCustomization.override_default_style(session[:preview_style]) %>
      <%=stylesheet_link_tag "application"%>
    <%- end %>

    <%- if mini_profiler_enabled? %>
      <%- Rack::MiniProfiler.step "stylsheet" do%>
        <%= stylesheet_link_tag "admin"%>
      <%-end%>
    <%- elsif admin? %>
      <%= stylesheet_link_tag "admin"%>
    <%-end%>
    <%=SiteCustomization.custom_stylesheet(session[:preview_style])%>