By default Qt Creator ignores the GTK+/Qt theme and uses its own light theme called "flat." This theme looks very ugly compared to the rest of the system and icons don't show up since Solus uses an icon theme with white symbolic icons by default.

Changing the default theme to "flat-dark" makes it integrate much better into the system.

This is set per-user at ~/.config/QtProject/QtCreator.ini like this:

[core]
CreatorTheme=flat-dark

Not sure how it can be set system-wide though.