Basically can there be a way to contribute to the AppThemeLookup's used by the XspPage?
It is annoying to install something like Bootstrap plugin and then not be presented with the themes that can be used. If a plugin can contribute to this lookup then that annoyance will go away.
Please see my blog entry
and also proposed solution
(obviously would need to be made more robust and IBM hardened by smarter programmers than me :)