Fix hover style for Colored Background and Colored Text modes

[jjroelofs]

Summary

• Removes transparent and inherit fallback values from base hover var() calls in dxpr-theme-navigation-colors.scss
• When the CSS custom properties (--dxt-setting-menu-hover-background, --dxt-setting-menu-hover-text-color, --dxt-setting-dropdown-hover-background, --dxt-setting-dropdown-hover-text-color) are not set by the user, the declarations now become invalid at computed-value time (IACVT), allowing the mode-specific selectors (hover-background, hover-text, hover-opacity) to apply their correct fallbacks (e.g., var(--dxt-color-base))
• When a user HAS explicitly set these variables via theme settings, they still apply in all modes

Test plan

• Select "Colored Background" hover style in theme settings and verify menu items get a colored background on hover (should use the primary/base color)
• Select "Colored Text" hover style and verify menu item text changes color on hover (should use the primary/base color)
• Select "Opacity" hover style and verify it still works (opacity change on hover)
• Set a custom hover background color in theme settings and verify it applies regardless of hover mode
• Test dropdown menu hover styles behave correctly in all modes
• Verify mobile menu hover styles are unaffected

Fixes: #785