GTK is so shitty to use that they created Vala to make it usable.
it kinda seems you’re not supposed to customize it
This is usually a good thing. More recent native frameworks like Qt QML do work more or less like designing web apps though, if that is what you want to. Qt Widgets itself provides a uniform look on platforms.
And vaguely related: Sciter, a little known but widely used framework that is a few megabytes and is similar to electron for design but very lightweight. Far older than it, too.
GTK is so shitty to use that they created Vala to make it usable.
This is usually a good thing. More recent native frameworks like Qt QML do work more or less like designing web apps though, if that is what you want to. Qt Widgets itself provides a uniform look on platforms.
And vaguely related: Sciter, a little known but widely used framework that is a few megabytes and is similar to electron for design but very lightweight. Far older than it, too.