Dark mode after refreshing page (#430)

### Description
This pull request introduces an enhancement to the existing application
by adding persistence to the dark mode feature. Currently, when the page
is refreshed, the dark mode setting reverts to the default light mode.
With this enhancement, the dark mode state will be maintained even after
refreshing the page.

### Changes Made
1. Added a new setting in the application to store the user's preference
for dark mode.
2. Implemented functionality to persist the dark mode preference in
local storage.
3. Modified the page initialization logic to retrieve the dark mode
preference from local storage and apply it on page load.

src/frontend/src/components/dropdownComponent/index.tsx

@@ -25,7 +25,9 @@ export default function Dropdown({
           <>
             <div className="relative mt-1 w-full">
               <Listbox.Button className="relative w-full cursor-default rounded-md border border-gray-300 bg-white dark:bg-gray-900 py-2 pl-3 pr-10 text-left shadow-sm focus:border-indigo-500 focus:outline-none focus:ring-1 focus:ring-indigo-500 sm:text-sm">
-                <span className="block truncate w-full">{internalValue}</span>
+                <span className="block truncate w-full dark:text-gray-300">
+                  {internalValue}
+                </span>
                 <span className="pointer-events-none absolute inset-y-0 right-0 flex items-center pr-2">
                   <ChevronUpDownIcon
                     className="h-5 w-5 text-gray-400"

src/frontend/src/contexts/darkContext.tsx

@@ -13,13 +13,16 @@ const initialValue = {
 export const darkContext = createContext<darkContextType>(initialValue);
 
 export function DarkProvider({ children }) {
-  const [dark, setDark] = useState(false);
+  const [dark, setDark] = useState(
+    JSON.parse(window.localStorage.getItem("isDark")) ?? false
+  );
   useEffect(() => {
     if (dark) {
       document.getElementById("body").classList.add("dark");
     } else {
       document.getElementById("body").classList.remove("dark");
     }
+    window.localStorage.setItem("isDark", dark.toString());
   }, [dark]);
   return (
     <darkContext.Provider

src/frontend/src/pages/FlowPage/components/tabsManagerComponent/index.tsx

@@ -116,7 +116,7 @@ export default function TabsManagerComponent() {
           </button>
         </div>
       </div>
-      <div className="w-full h-full">
+      <div className="w-full h-full dark:bg-gray-800">
         <ReactFlowProvider>
           {flows[tabIndex] ? (
             <FlowPage flow={flows[tabIndex]}></FlowPage>