Fix light mode, top nav external links, etc:

- Top nav external links now open in a new tab as intended (not specific
  to light mode)
- Consistently adjust opacity (not placement) of background images in
  mobile; lighter in light mode for legibility.
- Add a subtle white "shadow" to eyebrow text in light mode to improve
  legibility
- Fix focus styles for buttons & multicode tabs in light mode
- Make all buttons sized the same as primary buttons by default
- Fix typo that caused the "Reset" button on get-started to be unstyled
- Refactor theme switcher JS for simplicity.
- Fix issue with theme select icon not being visible in some cases.
- Update color of ✅ icons in interactive tutorial breadcrumbs

template/component-top-nav.html.jinja

@@ -42,14 +42,14 @@
             {% endif %}
             {% set _ = printed_groupings.append(link.top_nav_grouping) %}
             {% endif %}
-            <a class="dropdown-item{% if currentpage == link %} active{% endif %}{% if "//" in link.html %} external-link{% endif %}" href="{{link.html}}">{% if link.top_nav_name is defined %}{{link.top_nav_name}}{% else %}{{link.name}}{% endif %}</a>
+            <a class="dropdown-item{% if currentpage == link %} active{% endif %}{% if "//" in link.html %} external-link{% endif %}" href="{{link.html}}"{% if "//" in link.html %} target="_blank"{% endif %}>{% if link.top_nav_name is defined %}{{link.top_nav_name}}{% else %}{{link.name}}{% endif %}</a>
           {% endfor %}
           </div><!--./col-->
         </div><!--/.dropdown-menu-->
       </li>
       {% elif not top_page.top_nav_omit %}
       <li class="nav-item">
-            <a class="nav-link{% if "//" in link.html %} external-link{% endif %}" href="{{top_page.html}}"><span>{{top_page.name}}</span></a>
+            <a class="nav-link{% if "//" in link.html %} external-link{% endif %}" href="{{top_page.html}}"{% if "//" in link.html %} target="_blank"{% endif %}><span>{{top_page.name}}</span></a>
       </li>
       {% endif %}
       {% endmacro %}