increase_contrast.html

<button onclick="increaseContrast()">Increase Contrast</button>

increaseContrast.js

// Check local storage to see if the key 'contrast_increase' has been set to 'true'
window.onload = function() {
    if (localStorage.getItem('contrast_increase') === 'true') {
        increaseContrast()
    } else {
        console.log('Do Nothing')
    }
};

// Set contrast hex code in local storage
function increaseContrast() {
    document.getElementById('body').style.backgroundColor = "#DDDDDD";
    localStorage.setItem('contrast_increase', 'true');
}