I'm responsible for one of the forks that has morphed FA considerably. The code is still available on GitHub.
One thing attracted me as a developer to FrontAccounting was the way in which the page views used ui functions to do all (well almost all) their rendering of html. Because of this, all that is required to change the markup for every view is the 4 (or so) ui files. To take advantage of current html markup frameworks, like Bootstrap or Material, the html does need to change. Supporting multiple markup systems is easy, it just needs different implementations of the 4 (or so) ui files. With a small amount of refactoring multiple markup systems can be supported at the same time. This is polymorphism using classes, a very ordinary object oriented programming concept.
The modifications really are quite minor, the new ui classes diff very well against the existing ui functions. Names are the same, parameters are the same so it is quite easy to keep the classes up to date. I recently brought the code up to date with 2.4.3 and Bootstrap 4.
If that PR were to make it in to the core it then make it possible to safely experiment with new markup for new frameworks without breaking anything. e.g. Support for Bootstrap 4 could be introduced while still having Bootstrap 3 be the default, and so on.
There are two other ways that themes can introduce themes based on modern markup frameworks like Bootstrap.
1. Change the HTML on the fly with Javascript. Change tags, add classes, remove classes and have a theme that works with the modified html.
2. Change the markup framework so it works specifically with FA's existing markup. I've seen this done by some FA themes. This is a lot of work essentially duplicating the work that went into creating Bootstrap (but easier - its based from it). This is difficult to do well I think, its also difficult to incorporate new features from the ui framework.
I'm available to help (at no cost) should the developers want to incorporate this feature.