Add missing "alt" attributes to img tags in theme-selector.html (#689)

Co-authored-by: eishundo <eishundo@users.noreply.github.com>

layouts/partials/navigators/theme-selector.html

@@ -4,17 +4,17 @@
   default-theme="{{ site.Params.darkMode.default }}"></div>
 <a class="nav-link dropdown-toggle"  href="#" id="themeSelector" role="button"
   data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
-  <img id="navbar-theme-icon-svg" src="{{ "/icons/moon-svgrepo-com.svg" }}" width=20>
+  <img id="navbar-theme-icon-svg" src="{{ "/icons/moon-svgrepo-com.svg" }}" width=20 alt="Dark Theme">
 </a>
 <div class="dropdown-menu dropdown-menu-icons-only" aria-labelledby="themeSelector">
   <a class="dropdown-item nav-link" href="#" onclick="enableLightTheme()">
-    <img class="menu-icon-center" src="{{ "/icons/sun-svgrepo-com.svg" }}" width=20>
+    <img class="menu-icon-center" src="{{ "/icons/sun-svgrepo-com.svg" }}" width=20 alt="Light Theme">
   </a>
   <a class="dropdown-item nav-link" href="#" onclick="enableDarkTheme()">
-    <img class="menu-icon-center" src="{{ "/icons/moon-svgrepo-com.svg" }}" width=20>
+    <img class="menu-icon-center" src="{{ "/icons/moon-svgrepo-com.svg" }}" width=20 alt="Dark Theme">
   </a>
   <a class="dropdown-item nav-link" href="#" onclick="useSystemTheme()">
-    <img class="menu-icon-center" src="{{ "/icons/computer-svgrepo-com.svg" }}" width=20>
+    <img class="menu-icon-center" src="{{ "/icons/computer-svgrepo-com.svg" }}" width=20 alt="System Theme">
   </a>
 </div>
 </li>