Application palette changes at runtime do not work for all widgets

When application palette is changed at runtime (rather than at startup), the palette changes are not propagated correctly to all widgets. For example `QSpinBox`, `QComboBox` and `QTreeView` and maybe some others are not correctly updated, while most widgets are updated fine.

See https://stackoverflow.com/questions/47971350/change-application-palette-run-time-without-restarting-the-application for source code to replicate this wrong behaviour. Observed on Windows 10, but I expect the same on other platforms.

For example I started the application with 'dark' fusion theme:
https://i.stack.imgur.com/YPLlI.png

But when I switch the palette to standard fusion 'light' scheme at run-time I get this:
https://i.stack.imgur.com/Rdp08.png