refactor(overlay): made opacity variable configurable

[alokVishu]

Description

Added !default to overlay opacity variable to make it configurable.

Motivation and Context

How Has This Been Tested?

IDK much about tests but I had to generate new snapshot to commit my changes and possibly all test ran successfully.

Markup:

$overlay-opacity: 32%; => $overlay-opacity: 32% !default;

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• [x] The PR title is no longer than 64 characters.
• [x] The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• [x] My code follows the code style of this project.
• [ ] I've added relevant changes to the documentation (applies to new features and breaking changes in core library)