You do not need CSS for proper high-DPI support, you just need a scale factor for your windows that your code doesn't ignore. AFAIK this is what Qt5 does. Also

You do not need CSS for proper high-DPI support, you just need a scale factor for your windows that your code doesn't ignore. AFAIK this is what Qt5 does. Also AFAIK Gtk+3 cannot do real scaling and instead pretends the display is always a low DPI device and applies integer scaling to everything - their "fractional scaling" is really rendering with integer scaling upwards to a bitmap which is then downscaled (AFAIK^2 this is only done on Wayland because they rely on the compositor for that even though there is nothing that forbids them doing it on X11 too - but i guess they need to push their awful tech somehow). This is where things such as "there are no half-pixels" come from when they talk about fractional scaling. FWIW personally i use the OneStepBack theme for Gtk3 because it makes the controls actually visible (especially your selection in menus, tabs, etc that isn't just a blue line beneath the control) and saves a bit of screen space.