In Proton Mail Version 4.0(new window) (beta) you can change your theme from default mode to dark mode. This version is available to all Proton Mail users, including those with a free account.
Follow the steps below to enable dark mode in Proton Mail.
1. Log in to your Proton account at mail.proton.me(new window) and go to Settings → Appearance.
2. Scroll down to “Themes” and select “Dark mode”. Your theme will update automatically.