title: Apply GTK3 theme systemwide
body: <h3>Task / Problem:</h3>
  <p>I have a self-created gtk3-theme in <code>~/.local/share/themes/mytheme/gtk-3.0/gtk.css</code>. I would like to apply that style systemwide. How would I do that?</p>
  <h3>System:</h3>
  <ul>
  <li>XUbuntu 19.04</li>
  <li>GTK version: 3.24</li>
  </ul>
  <h3>What I did / have:</h3>
  <ul>
  <li>Prepare <code>~/.local/share/themes/mytheme/gtk-3.0/gtk.css</code></li>
  <li>Created <code>~/.local/share/themes/mytheme/index.theme</code> with the following content:</li>
  </ul>
  <pre><code>Type=X-GNOME-Metatheme
  Name=mytheme
  Comment=mytheme
  Encoding=UTF-8
  
  [X-GNOME-Metatheme]
  GtkTheme=mytheme
  MetacityTheme=mytheme
  ButtonLayout=:minimize,maximize,close
  </code></pre>
  <ul>
  <li>Created <code>~/.local/share/themes/mytheme/gtk-3.0/gtk.gresource.xml</code> with the following content:</li>
  </ul>
  <pre><code>&lt;?xml version='1.0' encoding='UTF-8'?&gt;
  &lt;gresources&gt;
      &lt;gresource prefix='/usr/darktrick/gtk-3.0'&gt;
          &lt;file preprocess='to-pixdata'&gt;images/icon_close.png&lt;/file&gt;
          &lt;file preprocess='to-pixdata'&gt;images/icon_close_active.png&lt;/file&gt;
          &lt;file preprocess='to-pixdata'&gt;images/icon_maximize.png&lt;/file&gt;               
          &lt;file preprocess='to-pixdata'&gt;images/icon_minimize.png&lt;/file&gt;       
          &lt;file&gt;gtk.css&lt;/file&gt;
      &lt;/gresource&gt;
  &lt;/gresources&gt;
  </code></pre>
  <ul>
  <li>Tested css with GtkInspector (under <code>CSS</code>): worked fine</li>
  <li>Theme is available in GtkInspector under <code>Visual</code> to be applied manually.</li>
  <li><strong>edit:</strong> I have chosen mytheme through the WindowManager dialog.</li>
  </ul>