Adjust toggle dark mode button label to translatable label

inst/pkgdown/templates/header.html

@@ -75,7 +75,7 @@
                   aria-expanded="false"
                   data-bs-toggle="dropdown"
                   data-bs-display="static"
-                  aria-label="Toggle theme (auto)">
+                  aria-label="{{ translate.ToggleDarkMode }}">
             <svg class="bi my-1 theme-icon-active"><use href="#circle-half"></use></svg>
             <i data-feather="chevron-down"></i>
           </button>