ux: improve checkbox contrast in dark theme

[a2580vb]

Purpose

Improve checkbox contrast in dark theme.

Changes

1. src/Resources/Themes.axaml

Added checkbox-specific resources to both Light and Dark theme dictionaries:

• Color.CheckBox.Border: Set to #898989 (same as Border1) in the Light theme, and #A8A8A8 (a brighter border for better visibility) in the Dark theme.
• Brush.CheckBox.CheckedBackground: Remains Transparent in the Light theme, while using the system accent color in the Dark theme.
• Brush.CheckBox.CheckedBorder: Remains #898989 in the Light theme, while using the system accent color in the Dark theme.
• Brush.CheckBox.CheckedFG: Uses the system accent color (unchanged) in the Light theme, and white in the Dark theme.

Additionally, Color.CheckBox.Border is registered as the Brush.CheckBox.Border brush in the shared section, and the three checked-state brushes are defined directly as SolidColorBrush instances within each theme dictionary.

2. src/Resources/Styles.axaml

• Unchecked border brush: Changed from Brush.Border1 to Brush.CheckBox.Border.
• Added checked state style ^:checked: Uses themed background and border brushes when checked.
• Check mark icon: Fill changed from the template default Brush.Accent to Brush.CheckBox.CheckedFG.
• :focus-visible:checked style updated to use the same themed resources.

Result

• Dark theme: Checked state now uses an accent-colored background with a white check mark, and a brighter border, significantly improving contrast.
• Light theme: All values remain identical to before; no visual change.