Countries rest Api app with React, React Router, mobile first

Hi @tomsky90, nice job on the dark mode and filtering. I do recommend to setup prettier to format your code to make it look nicer and not worry about formatting code. Also the components need to be simplified more, for example:

const HomePage = ({
    error,
    setError,
    darkMode,
    countriesListData,
    countriesDataForDisplay,
    setCountriesDataForDisplay,
    selectValue, 
    setSelectValue,
    inputValue,
    setInputValue,
}) => {
    return ( 
        <div>
            <Form  
                setError={setError}
                darkMode={darkMode}
                countriesListData={countriesListData}
                countriesDataForDisplay={countriesDataForDisplay}
                setCountriesDataForDisplay={setCountriesDataForDisplay}
                selectValue={selectValue}
                setSelectValue={setSelectValue}
                inputValue={inputValue} 
                setInputValue={setInputValue}
            />

this is like saying in vanilla js land Form(setError, darkMode, countriesListData, countriesDataForDisplay, setCountriesDataForDisplay, selectValue, setSelectValue, inputValue, setInputValue), I as a developer of this re-usable component wouldn't want to deal with. It needs to be more generic of a form. For example:

<Form
isDarkMode={true} //the "is" tells me it's a boolean
data={[]}
onSubmit={() => {
// fetch, set your countries, set errors, and set load here
}}
onInputValueChange={() => {}}
onSelectValueChange={() => {}}

This way you can re-use the component in more than one place.

you might want to work on some hover effects for the cards and the back button but otherwise you have a good start to this