It looks like the anchor element isn’t being closed when the logo is blank.
<tr>
<td style="padding: 10px 10px; background-color: #<%= @header_color %>;">
<a href="<%= Discourse.base_url %>" style='color: #<%= @anchor_color %>'>
<%- if logo_url.blank? %>
<%= SiteSetting.title %>
<%- else %>
<img src="<%= logo_url %>" style="max-height: 35px; min-height: 35px; height: 35px;" class='site-logo'></a>
<%- end %>
</td>
</tr>