[4.7] Fix Style dialog dropdown items too dark on Windows (light mode) #32000

[pyonpyoco]

Resolves: #32000

ComboBoxDropdown.qml explicitly sets the ListItemBlank delegate’s normal background (normalColor: ui.theme.buttonColor), forcing items to be filled at all times.

This change removes that override so dropdown items use the default ListItemBlank styling (transparent idle background + theme-driven hover/pressed/selected states).

This keeps the change minimal and avoids broader dropdown/popup styling changes.
Tested on Windows 11 light mode (Chord Symbols / Measure Numbers / Repeats). Before/after screenshots attached.

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[zacjansheski]

dropdown is very dark on my Windows device

[pyonpyoco]

I tested Windows 11 theme settings and found the behavior depends on “Default app mode”.

• App mode Light: OK (matches the scope of this fix)
• App mode Dark (including Custom: Windows Light / App Dark): the Style dialog dropdown popup becomes very dark / low-contrast.

This seems like a separate dark-mode styling issue (popup/menu layer using a different palette/theme).

(Japanese UI labels: Light = ライト, Dark = ダーク, Custom = カスタム, Default app mode = 既定のアプリ モード)

[pyonpyoco]

In my previous test, I overlooked the MuseScore setting.
The previous results were based on Theme = Light.

Repro matrix (Windows 11): dropdown popup colors depend on both MuseScore “Appearance theme” and Windows “Default app mode”.

• MuseScore Theme Light + Windows app mode Dark → popup becomes too dark.
• MuseScore Theme Dark + Windows app mode Light → popup becomes too light.
• Theme matches (Light+Light, Dark+Dark) → popup looks correct.