Click on Your profile picture (bottom left) → Company settings → Project accounting → Appearance → Estimate template → Hover over the desired template → Edit. There you can edit the HTML or add custom CSS code.
If you click on Add from library you will be able to select different languages/templates and set the desired default one.
Tips & Tricks If you wish to have custom fonts you need to use Google fonts. Here’s an example of how to add the font in the CSS section:
@import url(https://fonts.googleapis.com/css?family=Tangerine);
.invoice { font-family: 'Tangerine', cursive; }