Step by Step
- Open your Protonmail account in your web browser, your Protonmail web-interface will appear
- Click “Settings” in your Protonmail web-interface
- Click the “Theme” button, by default a blank text field will appear
- Copy everything between BEGIN and END of the css code above, i.e. from this file cohibaSIGNUMws_min.css
- Paste the copied text into the blank text field
- Click the “Save” button below the text field
Currently there are two themes that I know of. But I am sure that someone can create their own.
Also there has been a pretty large update to Protonmail; for the release notes check out this page.