ProtonMail is a secure email service that allows you to customize the appearance of your email messages using CSS (Cascading Style Sheets). Here's how you can add CSS to your ProtonMail email messages:

Create a new email message or reply to an existing one.

Click on the "HTML" button in the toolbar at the bottom of the email composition window.

This will switch the email composition window to HTML mode. You can then add your CSS code to the email message by enclosing it in <style> tags.

For example, you can add the following CSS code to change the color of the email text:


  body {

    color: red;



Once you have added your CSS code, you can switch back to the normal email composition view by clicking on the "HTML" button again.

When you send the email, the CSS styles will be applied to the email message.

Note: Keep in mind that not all email clients support CSS, so the styles you apply may not be displayed correctly in all email clients. It's always a good idea to test your email messages in different email clients to ensure that they are displayed correctly.