I have a html page with two CSS files - one for a light theme and one for a dark theme. When I click the respective button, the theme changes to either light.css or dark.css. >
You can use a cookie, localStorage or sessionStorage where you keep the default theme and the choosen one. So when your page loads you have to read that info from cookie/localStorage/sessionStorage and to apply the chosen theme.
e.g when page is loading
let theme = localStorage.getElement('theme');
if(!theme){
theme = 'light';
localStorage.setElement('theme', 'light');
}
// and you use theme variable to load the light theme
e.g when the user changes the theme
localStorage.setElement('theme', 'dark');
theme = 'dark';