My web application has a dark theme and light theme. How should I decide which one should be enabled for first-time visitors?
2 Answers
Let the user choose
When the user opens the app for the 1st time, you can ask them to choose their preference. Give them the option to change it later on in settings.
Also, you can give them the option for it to change based on day/night time(if your application has this feature).
These days, it's not too hard to detect what the user's already chosen. This takes the burden of deciding off of you, and is likely to make a smoother experience for the user in general. (For example, when I'm using my OS and Creative Cloud apps in dark theme, then open the browser and get a face full of bright light, it's less fun.)
Here's a screenshot of how StackOverflow does it. They have a third option for "system setting", which is chosen by default. This mirrors pretty well the experience of many apps on phones, which were a little ahead of the curve on dark/light themes, I think.
If you must choose one or the other for some reason, I love sibert's suggestion in a comment of using the more accessible one using a measuring tool.