But how do you deal with icons that are not part of the Fontello font as shipped with Plone. In this case I want the Dropbox icon from the Fontello font...how to include it? Or is the general recommentation to use dedicated SVGs here?
The problem with Fontello is, its not a font as usal.It calls itself a "Icons font generator" and it's meant to create a custom font for each use. Maybe because as one big font it would be to large. So one picks out the icons needed and creates in fact a custom font to be used in the site. This was done for Plone too. So on download a name can be choosen for this custom font.
In Plone a major mistake happened: Instead of plone-fontello-icons or such a name, the font was just named fontello.
Also another mistake happened: In CSS all icons in toolbar or similar places, where other addons can hook in, are considered to come from this one fontello font.
So yes, the current situation is bad and I guess - whoever did this - the person did not understand how fontello works and its consequences for addons.
This need to be fixed, but this is difficult w/o breaking current addons.
Creating a font for every package appears over the top. I need exactly one icon for the toolbar for every add-on...this is not approachable and maintainable...guess SVGs are the way to go...everything else is too complicated (as too many other things in Plone 5).
As you can also read from the comments of Volker: the road taken here for icons is broken and not approachable. There is no way to justify some hours of work to a customer just for adding/changing an icon...technical perhaps a clean solution, for real world usage: over-designed, too many steps, error-prone and therefore unusable.
The point is that icons are done with css. CSS icons have been done for many years in web development. You can achieve this with css through font icons, svg icons or plain image icons. You can reference actual files or better yet, inline them with base64. Font icons are nice because their color can be changed with css; however, again, you do not need to use them.
This is not much different than Plone 4 which used image sprites and css for content type icons.