Style All WidgetsΒΆ
To apply the same color style to all widgets use the All tab.
Pick a Text Color or Background Color for the state you want that color to be. The color Selector will popup and you can pick the color you want. Then click on OK. If you click on Cancel it will remove any color previously selected.

The color selected will show up in the box next to the button.

Note
Most widgets use all 4 states
To apply all the color styles selected click on Apply Color Style to All Objects.
Now if you view each tab you can see the colors applied as well as the ToolButtons on the top.

Also note the Stylesheet tab in each widget tab has the QSS stylesheet code used so you can copy and paste from there.
You can launch a sample GUI to see all the stylesheet changes at the same time.

Note
If you change a style you have to close the sample window and reopen it to see the changes.