Yes, that's the look of the Fusion widget style. The light look is lacking some contrast. Here you can see the available styles:
Qt Widget Gallery. The problem is that only Fusion is available on all platforms and each style has a little bit different layout metrics, so I would have to make widget layouts that fit all possible styles, which can be a lot of fiddling around. Havin one style makes it easier.
I could use a checkbox instead of a push button, but it needs addditional horizontal space for the label. I can also modify the style and change the pressed state color. Perhaps that's what I will do.
However, on my monitors the pressed state is quite distinguishable, but I have IPS panels on both my desktop and my laptop. I know that TN panels are lacking contrast and are fading colors.