In 1999, I built a completely custom Win32 GUI for a (brandable) chat application - that was the product the company I worked for was selling.
Pure C and C++. It was a 32-bit app, not 64 bits. And we felt bad for it being 250KB single executable (skin included in resource section) and not “150KB or less” as was our initial target. But making it accessible and fully skinnable/themable/l10n/i18n did add quite a bit which we hadn’t realized when thinking 150KB was realistic. But it’s not like we resorted to any crazy tricks; that was just the stripped linker output.
It was still a reasonable download at the dominant dial up internet at the time (28.8Kbps IIRC)
Sure, modern tools add a lot of cruft that you’d have to work hard to reduce, but 40MB. Oh man.
It is perfectly fine to build a 150KB or less executable today. But then OP decided they don't want to write paint functions for custom GUI and insist on finding a library to do so, I guess that's not what you did in 1999, right?
So, because pervasive theming/branding was a hard requirement, and i18n a softer one (every skin has to only support one locale), i had to abandon FLTK. But FLTK did spoil me by providing seamless flicker free double buffering out of the box, which I insisted on implementing in our gui.
For those of you who don’t remember - Win32 flickered badly on almost all screen updates, window resizes, etc — I think that was generally true of most apps until 2010 or so. FLTK was essentially the only GUI toolkit at the time (1999) that had no flicker out of the box.
Given that it’s windows, it might still run today, so it likely is still possible
I still use Winamp, and essentially in its 2.x mode.