Adding a dark mode is basically adding a theme. The principles are the same for adding a light mode to a dark website or alternative styling based on user-defined variables, the time of year or holidays.
I added theming with a mix of and CSS. In this post I’ll go step by step into the details of how I did it and what I learned.
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Setup
The themes are activated by CSS classes on the root element. When the page is loaded, I want to apply the theme that most likely suits the visitor (you!) best. After all, most people don’t like configuring websites before they can read a blog post, so the the whole theming feature would likely remain unused otherwise. So I have to make a guess about what the visitor wants and expects. I do that in this order:
- I assume people don’t want the theme to change when they navigate between pages. So if the page loaded isn’t the first page they visit, I want to use the theme that was used before.
- If it’s the first page they view on my site, their browser may be able to tell their preference.
- If no preference is available, we can base the choice based on whether it’s day or night.
Every time a page is loaded, in the current or a new tab, it checks if a theme was set previously. Because the preference for a light or dark theme can change during the day, with every change, I add a time stamp to the saved setting. Only when the state was saved less than two hours ago, it’s applied:
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