Use the default accent color if the user removes value from textbox (fixes #368)

[vraravam]

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Description of Change

Use the default accent color if the user deletes the value from the text box in the preferences screen.

Motivation and Context

Fixes #368

Screenshots

Checklist

• My pull request is properly named
• The changes respect the code style of the project (npm run prepare-code)
• npm test passes
• I tested/previewed my changes locally

Release Notes