Thought I'd post this 'gotcha' that may be relevant if you have HTML5 tripane targets that you are migrating from Flare 11 to 12.
The issue was that since migrating from Flare 11, my toolbar buttons disappeared.
I have a 'Global' project that contains all the common 'resource' files, including the skins, but no 'content' topics.
This is imported into several child projects. All of these exhibited the same problem when rebuilt in Flare 12 - no toolbar buttons.
The issue was that the .flskn files were all based on an import from Flare 11, and there had been no reason to reimport them since upgrading to 12 as there had been no material change in any of the Global project files.
The solution was to rebuild the Global 'master' project in Flare 12 and reimport the (otherwise identical) skin into the child projects.
Hope that helps somebody! - Robin.