Add a new mid-grey color variable with dark/light theme variants

Issue

The UI color system is missing a grey that feels like a mid-grey both in light and dark themes.

This would be useful for cases in illustrations that are affected by light/dark/high-contrast mode where we want to "dim" something without relying on opacity or the score color variables (which do not change between light/dark mode).

As an example: the X in the Open/Atonal element in Palettes > Key signatures used to use the score "Formatting color" variable, which became an issue when we made it pink. Now, it's grey again but uses a score variable added in this PR, so it can't be different in light/dark/high-contrast modes.

Right now, HC dark mode looks like this:

The grey X should probably just take the same color as HC text in this case.

Recommendation

Light mode/dark mode: make it the as the default frame grey, but don't change it if the user changes the frame color in Preferences > Advanced
High contrast light mode: follow FONT_PRIMARY light mode
High contrast dark mode: follow FONT_PRIMARY dark mode