ComboboxControl: Replace undefined variable usage with color-mix for disabled selection

[yogeshbhutkar]

What and Why?

Closes #69593

This PR:

1. Removes the usage of the --wp-components-color-accent--rgb property as the rgb variant does not exist.
2. Removes the usage of $components-color-accent-transparent-40 SASS variable in favor of color-mix().
3. The background for disabled and selected/focused options on dark themes is now visible and looks as expected on dark backgrounds.

How?

The corresponding scss file is updated to reflect the above changes.

Testing Instructions

1. Run the storybook dev server. (npm run storybook:dev)
2. Navigate to ComboboxControl.
3. Toggle the theme to dark mode.
4. Confirm the background is visible for selected/focused disabled options.

Testing Instructions for Keyboard

Same.

Screenshots

Before	After

P.S. Please note that a tinted background (although faint) has been added to the first (disabled) option. The second option may appear invisible due to the lack of theming support, which will be addressed once theming is implemented for the component. (Ref. #69593 (comment))

[github-actions]

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Co-authored-by: yogeshbhutkar <[email protected]>
Co-authored-by: ciampo <[email protected]>
Co-authored-by: mirka <[email protected]>

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[ciampo]

Code changes LGTM 🚀 Thank you for the fix!

A nice follow-up would be to make the whole component themable

[yogeshbhutkar]

Thanks for the review, @ciampo.

A nice follow-up would be to make the whole component themable

Will follow-up on this shortly!