Add dark theme by alessandro.g89

Source: https://userstyles.org/styles/122502/syncthing-dark

gui/default/syncthing/core/aboutModalView.html

@@ -14,6 +14,7 @@
       <ul class="list-unstyled three-columns" id="contributor-list">
         <li class="auto-generated">Aaron Bieber</li>
         <li class="auto-generated">Adam Piggott</li>
+        <li class="auto-generated">Alessandro G.</li>
         <li class="auto-generated">Alexander Graf</li>
         <li class="auto-generated">Anderson Mesquita</li>
         <li class="auto-generated">Andrew Dunham</li>