Anchor overlay color should take into account paper color

Issue type

UI bug (incorrect info or interface appearance)

Description with steps to reproduce

This is about the colored stripes shown on the score when repositioning dynamics/lines using the Shift + Arrow key shortcuts introduced in 4.4.

They are meant to look like they are semitransparent overlays beneath the score. This is working correctly when the score page color (Preferences > Appearance) is the default, but when the score is inverted or set to another color, the overlay still looks as if it had a white background behind it.

@mike-spa implemented this by running a calculation using the page color, but I'm guessing that was hard-coded instead of pulling from the page color set in preferences.

However, simply using the same transparency values on a dark background renders the stripes nearly invisible (see screenshot below). So if we want to do this right, we should use a smarter color calculation to make stripes brighter on backgrounds darker than a certain value.

Supporting files, videos and screenshots

Example on inverted background

https://github.com/user-attachments/assets/b288d728-ec06-4799-9eba-d8ea1366df5a

Example on color background

https://github.com/user-attachments/assets/9a12ea4d-12a9-4fde-b5e3-3c7443d2b451

What is the latest version of MuseScore Studio where this issue is present?

Nightly

Regression

No.

Operating system

macOS 14.5

Additional context

No response

Checklist

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• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible