[WC-3052] DataWidget: Styling variables cleanup

[yordan-st]

Pull request type

Refactoring (e.g. file rename, variable rename, etc.)

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Description

Consolidated all SCSS variables from individual files into variables.scss to eliminate duplicates and create a single source of truth for theming, adding !default flags for easier overrides.

What should be covered while testing?

• All data grid components render with correct colors, spacing, and styling
• Filter dropdowns, buttons, and icons display properly
• Hover states, selected states, and focus styles work as expected
• Variables can be overridden using !default mechanism
• CSS custom properties update correctly at runtime
• No duplicate or conflicting styles applied