There is a useful discussion going on about Qt’s limitations.
I thought that one of solution would be to impliment QSS available by default.
Before,I remembered when make some Krita themes for Style Sheet Loader Extension Plugin
Because, I disstatisfication of Krita UI elements were coloring contrast is small and simimar(Dark tab hilight had been same contrast,very hard to saw ) ,and UI elements packed too closer to the edge of the window.
These theme files called to qss file(QSS: Qt style sheet )
It has similar syntax of Cascade Style Sheet
If Qt version up the QSS customization possiblity must be increase.
About Qt Style sheet Limtation
(A followings as far as I knowledge over trying and errors 
)
It could
- customize frame color (and scroll grabber)
- Customize background color of heaader and status bar
- Tab style (round border,background color,gradation,padding etc)
- Icon button border on/off
- Tab hilighit color
- Pane color(background and text color)
- General Padding
It couldn’t
- Bold UI changes, direct interference with existing UI
- Tool/button icon color change freely (SVG file ,Currently Avaiable black and white colors only)
- Padding and margin of palette window (setContentsMargins(), Python function exist for it but for global).
Furthermore, if want to change the UI behavior,it may need to modify the internal code or by Python…
PluginDevTool is helpful for exploring Krita UI