Easier theming e.g. for Bootstrap UI framework

Currently all Decidim’s UI functionality is quite attached to the underlying UI framework, Foundation. However, in some cases it seems it would be useful to be able to use other UI frameworks with Decidim, too. For example at City of Helsinki they have developed their own UI framework that gives developers easy access to the City of Helsinki branding assets and make it easy to apply to new projects. For existing projects I believe the end goal at Helsinki would be that they can make changes to the UI library in one place and eventually the change will roll out to all the services they have. Helsinki has a bunch of digital services you can check at (in Finnish): https://digi.hel.fi/projektit/ I have attached a more detailed explanation in this proposal about the problems with the current version of Decidim to be able to easily swap the UI framework.