Fix issues with "Hide labels from issue board cards"

What does this MR do?

This MR fixes a couple of issues raised in the already merged MR: !18533 (merged)

• A SCSS variables are reused
• The new breakpoint for the boards search bar column layout is created for between 768px and 992px because the new "Show labels" toggle pushed form controls to its right off the viewport around 768px-992px viewport width

Screenshots

Screenshot of viewport at 983px width: (taken for commit 832d4896)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team