Improve consistency of navbar icon declaration

[markdumay]

Description

The file scss/_variables.scss defines several embedded svg images. The svg definitions use parameterization to support Bootstrap's color system. The declaration of $navbar-light-toggler-icon-bg is inconsistent with the other declarations, as it uses #{rgba($body-color, .75)} instead of referring to a predefined variable.

Motivation & Context

This PR introduces a new variable $navbar-light-icon-color to improve consistency. The existing variable $navbar-light-color is kept intact to avoid introducing unwanted side effects.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-37720--twbs-bootstrap.netlify.app/

Related issues

N/A

[mdo]

I had done the separate fill on purpose, but I’m fine having a variable for it. Thanks!