Comment by Marco Alka on "What are some ways of implementing a togglable “dark mode” on a website?"

I usually architecture theming into the CSS from the beginning, by just following Harry's BEMIT guide.

I assign a "root" theme class to the body element, like t--light
Then, every element with a theme (even if only one theme is planned) will get a t- (theme) CSS class. For example t-menu__item--active
To make it work, I prefix the CSS selectors with the root theme class. If I want to add a new theme, all I have to do is copy all theme related CSS classes, change the prefixed CSS theme class and do the actual theming
Switching the theme is as easy as removing and adding a CSS class in the body tag (either on the server or the client side).

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