Not a full blown UI kit but lots of cross platform goodies (C++)

https://www.cockos.com/wdl/

https://github.com/justinfrankel/WDL

WDL Virtual Window system:

    Allows the building of dynamic, complex UIs within OS hosted windows
    Included controls for text display, combo boxes, buttons, sliders, list boxes
    Supports full transparency, overlays, controls with shadows/highlights outside of their range
    Fully themeable (uses LICE for compositing)