More modular scss files

[danielr18]

Is your feature request related to a problem? Please describe.
Describe alternatives you've considered
I wouldn't like to have to duplicate all the app.scss code to just delete the Lato font import, or if I have light/dark themes in the site, having to either import the complete app.scss under different selectors or duplicate the rules related to colors (which would lead to having to keep a look on styles updates if you update the component)

Describe the solution you'd like
If app.scss is split in 2 files: non theming related and theme related, you could have multiple themes with less css output, and without needing to duplicate the stylesheet (leading to easier upgrades later on). The default font family could be set in the default scss file like calendar.scss, but not in the modular files, to avoid downloading a font you might not want to use.