Tutanota does not give much options for editing their interface. Here is a Chrome (and Brave) extension that allows you to tweek the fonts using CSS.
The changes I have made are to reduce font size in the email listing (making it easier to scan email) and to change the colour of unread email so they stand out more.
This is a better approach than using external website editors as we can ensure we maintain the security of the system.
Please note if Tutanota upgrades, this extension may cause issues so switch off the extension if this happens.
This is based on this tutorial: Create a Chrome extension to modify a website’s HTML or CSS