Instead of having two different types of effects (hover had a different
color, and active had a 3d-effect simulating a pressed button), we now
have an increasing activation-level. That means the button goes a bit
dark for hover, and then even darker when pressed.
There is also a variant that goes lighter for each activation level,
that can be used when the initial color is dark.
With this change, we can get rid of special :hover and :active styling
for the connect button and the control bar buttons. We can use the same
activation level principle for all buttons.
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