Since v1.16, the encrypted mail service now allows themes using the Cascading Style Sheets! So feel free to pick one and use it. Now with v2.0 comes updated theme goodness.
Simply pick one from the themes folder and copy the code over to ProtonMail by heading over to Settings, click the Appearance tab and paste the code under the CSS Theme Code section.
I would gladly post any themes made by you in order to make this repository grow so please feel free to send your themes over and be part of this project.