Thanks as well :)

Are there any special steps required to land it on the invent repository?

shorten selfTimerIcon color definition

One thing that should probably be considered is how to handle toolbars that are not at the top, but on the sides or the bottom of the window, as well as any combination of those. The possible cases seem to be quite tricky.

I think the icon for an intermediate state should visually be like a fraction of the checked state. That would make it visually clear which state corresponds to more selected sub-items.
What I mean could be roughly described as: the intermediate state icon should not contain filled pixels where the checked state has none. And also the checked state icon should have more filled pixels in total than the intermediate one.

In T10891#185961, @onvitaik wrote:

Since I saw those crude mockups, I've been changing it with the suggestions I wanted to make plus others. It takes some pages taken from how other DEs do their things and some of my own preferences, while at least still trying to not deviate too wildly from the Plasma look. I've also combined it with some suggested changes to the current system settings application.

{F6844666}

Maybe some kind of greying out or adding transparency would be an idea for locked tracks, might as well work for muting audio or hiding video. Greyed out elements are common in GUIs to indicate something is disabled or can not be changed.