[REF] web_editor: editor UI style and stability improvements

* Review style of some UI components
* Rename some content
* Define a font-family less variable to be used to avoid that themes
customizations affect the UI. Note: theme should ideally not redefine
the font family of the body but of the #wrapwrap element but this is
done for safety.
* Overwrite other theme's customisations
* Minor style fixes