They look pretty nice. Some notes based on my experience.
The single-color icons in the wbar mockup are hard to distinguish from each other. Try looking at them with your side eye, you can't tell them apart - this is a good way to simulate reduced vision. Even people with good vision, looking straight on, take longer to tell them apart, which reduces usability. You often have to use them by remembering their position, which is not what you want. Even some big-name apps fail in this point, some versions of 3ds Max have a dark theme where all the button icons are indistinguishable from each other, you have to highlight each one and wait for the tooltip to make any sense of it.
The lack of edges and guiding colors in some elements, the so called "flat design" that is in fashion, likewise reduces usability. In both the dark and light mockups, look at the search text field - you can't tell it's a text field or a background element. It doesn't look clickable, or text-enterable. The lack of borders in the scrollbar buttons makes them harder to hit.
FLTK is fairly easy to customize on colors, you can define what colors you'd like for all of its defaults in your .Xdefaults file. If the app overrides a color, then you'd need to edit the app's code. Other aspects of appearance would also need code changes.