KDE/kwin: Workaround for 1.2.1-beta.0 Config Sliders and Feehand Tool

I had reported this here:

But is duplicated with this:

If you have any of the following problems using KDE:

1- Configuration sliders does not work in the GUI
2- When drawing on the canvas, only straight lines are drawn. The first point of that line corresponds to when you start to draw, and the last point of the line to when you release the button. All this in freehand painting mode § selected.

This is related to “oxygen-gtk” "engine in KDE. A workaround for “1” is go to fullscreen and un-fullscreen. But this not solve “1”. So, the solution for MyPaint is to change the oxygen-gtk engine by other from KDE System Settings, apps style.

A summary of keywords that Spanish speakers can find this:
Si usando MyPaint en KDE tienes algunos de los siguientes problemas:
1- Las perillas deslizadoras de configuración de la interfaz gráfica no funcionan
2- Solo lineas rectas son dibujadas en modo de dibujo a mano alzada §

Reemplazar el motor oxygen-gtk por otro motor desde las preferencias del sistema, Apariencia de las aplicaciones > Apariencia GTK.

Hi again. I had forgotten to mention this that is important.
If you do not want to change the theme for all your GTK applications when you use KDE, you can launch MyPaint (or any GTK application) as follows using a GTK theme (Adwaita or Adwaita-dark for example):

GTK_THEME=Adwaita mypaint
GTK_THEME=Adwaita-dark mypaint

Remember that for use Adwaita in Kubuntu, you must install ‘gnome-themes-standard’ package.