Add support to customize bulma styles

[facundoolano]

The current CSS takes vanilla bulma and customizes it by overridding styles, in some cases by hardcoding and duplicating variables that could be reconfigured if we were building bulma from its Sass source.

• Add support to build bulma from Sass (the compiled CSS shuold be checked in, the Sass customization shouldn't add setup overhead to run the app).
• Replacce hardcoded values to bulma variable customizations.
• Review main.css to look for potential improvements or further bulma customizations.