Fix global search style problem

app/assets/stylesheets/startup/startup-dark.scss

@@ -1831,6 +1831,9 @@ body.gl-dark .navbar-gitlab .search form:active {
   background-color: var(--gray-100);
   box-shadow: inset 0 0 0 1px var(--blue-200);
 }
+body.gl-dark .navbar-gitlab .search form .search-input {
+  color: var(--gl-text-color);
+}
 
 body.gl-dark {
   --gray-10: #1f1f1f;

app/assets/stylesheets/themes/dark_mode_overrides.scss

@@ -122,6 +122,10 @@ body.gl-dark {
           background-color: var(--gray-100);
           box-shadow: inset 0 0 0 1px var(--blue-200);
         }
+
+        .search-input {
+          color: var(--gl-text-color);
+        }
       }
     }
   }

ee/app/assets/stylesheets/startup/startup-dark.scss

@@ -1831,6 +1831,9 @@ body.gl-dark .navbar-gitlab .search form:active {
   background-color: var(--gray-100);
   box-shadow: inset 0 0 0 1px var(--blue-200);
 }
+body.gl-dark .navbar-gitlab .search form .search-input {
+  color: var(--gl-text-color);
+}
 
 body.gl-dark {
   --gray-10: #1f1f1f;