In the earlier days, both Qt and gtk quickly jumped on the theme train, where you could customize the looks of widgets heavily, and IIRC FLTK never caught up there. I think the default L&F can compete with default gtk1/2 or Motif-ish Qt, but you couldn't go up from there or try to have a common look & feel between toolkits.
These days, that has gone one step further. Where a lot of people consider anything with "old-school" widgets to be ugly and outdated, and a more web-like cards/list-view based approach with highly customized and animated UI elements.
Qt with its QtQuick/QML parts seems quite competitive there, not sure how good Fltk 1.4 is here.
(Just repeating the way I see the "state of the art" here, I personally don't like that "proper" GUIs are now retrocomputing)