add all the react custom hooks

src/hooks/useCountries.jsx

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+import { useEffect } from "react";
+import { useState } from "react";
+
+function useCountries() {
+  const [countries, setCountries] = useState([]); 
+  useEffect(() => {
+    const requestCountries = async () => {
+      const countriesRequest = await fetch(
+        "https://restcountries.com/v3.1/all"
+      );
+      const countriesResponse = await countriesRequest.json();
+      setCountries(countriesResponse);
+    };
+    requestCountries();
+  }, []);
+  return [countries];
+}
+
+export { useCountries };

src/hooks/useCountryInfo.jsx

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+import { useEffect, useState } from "react";
+
+function useCountryInfo(countryName) {
+    const [countryInfo,setCountryInfo] = useState(null);
+    useEffect(() => {
+        let isRequestCanceled = false;
+        const getCountryDetails = async () => {
+            if(!isRequestCanceled){
+                const COUNTRY_API = `https://restcountries.com/v3.1/name/${countryName}`;
+                const countryRequest = await fetch(COUNTRY_API);
+                const countryResponse = await countryRequest.json();
+                setCountryInfo(countryResponse[0]);
+            }
+        }
+        getCountryDetails();
+        return () => isRequestCanceled = true;
+    },[countryName]);
+    return [countryInfo,setCountryInfo];
+}
+
+export default useCountryInfo;

src/hooks/useDarkMode.jsx

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+import { useEffect, useState } from "react";
+
+function UseDarkMode() {
+    const userDefaultTheme = (window.matchMedia("(prefers-color-scheme:dark)").matches) ? "dark" : "light";
+    const userSetTheme = localStorage.getItem("rest-country-theme");
+    const currentTheme = userSetTheme ?? userDefaultTheme;
+    const [theme,setTheme] = useState(currentTheme);
+
+    useEffect(() => {
+        document.documentElement.setAttribute("class",theme);
+        localStorage.setItem("rest-country-theme",theme);
+    },[theme]);
+
+    return [theme,setTheme];
+}
+
+export default UseDarkMode;