Improve visibility of muted parts

Muted parts are drawn with a white background. That looks fine in a light theme like mine:

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But it doesn’t look good in a dark theme (which is the default), because they stick out. Muted parts shouldn’t stick out IMO.

Muted parts don’t show their part colours. So a part that has a specific colour…

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…just gets the colour of a muted part when muted:

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Also, it’s very hard to tell if a part has been muted if it is selected. This part is muted and selected:

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And that’s how it looks if it’s selected, but not muted:

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All of these problem could be simply solved by using opacity for muted parts. Part colour and selection colour are applied as if it wasn’t muted, and then the whole part is draw with an opacity of, let’s say, 42%.