[QtGUI] Inconsistent colors when palette is used

Palette is giving inconsistent colors through different platforms (dark and light theme both).

Colors used to test and compare palette colors: black, palette.light, palette.midlight, palette.dark, palette.mid, palette.shadow. 

Pallette test application has also been attached.

Observations:

Screenshots of Windows, MacOS and Linux(both dark and light theme) have been attached for reference. 

1. MacOS dark theme - palette.dark is lighter than all other colors used
2. Linux dark theme - palette.light is same as palette.mid (looks dark only)
3. Windows dark theme - palette.dark and palette.mid are exactly same colors
4. Linux light theme - palette.light, palette.midlight and palette.mid are exactly same colors.

Also the difference in color shades among different platforms is very high (light and dark respectively).