Improve colours and fonts in the editor

[emilioSp]

The editor appaerance is customizable.

We should improve the color scheme and the font in order to harmonize it on the page.

cc: @Fupete

[Fupete]

This could be usefull, I will look into it next week.

Maybe in pair with this to try the json of the theme under development.

cc @emilioSp

[Fupete]

@emilioSp assign the issue to me for now, as a reminder! :-)