discourse/app/views/application/_header.html.erb

<%= replace_plugin_html('server:simple-header') do %>
  <header class="d-header">
    <div class="wrap">
      <div class="contents clearfix">
        <div class="title">
          <a href="<%= path "/" %>">
            <%- if application_logo_url.present? %>
              <picture>
                <%- if application_logo_dark_url.present? %>
                  <source srcset="<%= application_logo_dark_url %>" media="(prefers-color-scheme: dark)" />
                <%- end %>
                <img src="<%= application_logo_url %>" alt="<%= SiteSetting.title %>" id="site-logo" />
              </picture>
            <%- else %>
              <h2 id='site-text-logo'><%= SiteSetting.title %></h2>
            <%- end %>
          </a>
        </div>
        <div class="panel clearfix">
          <%- unless current_user || local_assigns[:hide_auth_buttons] %>
            <span class='header-buttons'>
              <%- if can_sign_up? %>
                <a href="<%= path "/signup"%>" class='btn btn-primary btn-small sign-up-button'><%= I18n.t('sign_up') %></a>
              <%- end %>
              <a href="<%= path "/login"%>" class='btn btn-primary btn-small login-button btn-icon-text'><%= SvgSprite.raw_svg('fa-user') %><%= I18n.t('log_in') %></a>
            </span>
          <%- end %>
        </div>
      </div>
    </div>
  </header>
<% end %>