Adjust Font Size in 2017 Default Theme
The font size for the new (2017) Default theme is a bit small for some so here's a style sheet that will adjust things for you and get you started for further customizations.
The download (found here) unzips to a sa_templates folder, which you can add directly to your (child) theme. Once it's added to your theme the CSS will automatically get picked up and used alongside the theme's stylesheet(s).
For more details on custom stylesheets, including a way to add a separate stylesheet for each estimates and invoices, please read the customization documentation.
I hope that all makes sense, if not please contact support.