fix: setting z-index in themeVariables

[svenvoskamp]

Description

Please include a brief summary of the change.

Type of change

• Chore (non-breaking change that addresses non-functional tasks, maintenance, or code quality improvements)
• 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)

Associated Issues

For Linear issues: Closes APKT-3673
For GH issues: closes #...

Showcase (Optional)

If there is a UI change include the screenshots with before and after state.
If new feature is being introduced, include the link to demo recording.

Checklist

• Code in this PR is covered by automated tests (Unit tests, E2E tests)
• My changes generate no new warnings
• I have reviewed my own code
• I have filled out all required sections
• I have tested my changes on the preview link
• Approver of this PR confirms that the changes are tested on the preview link

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	78.33%	34812 / 44442
🔵	Statements	78.33%	34812 / 44442
🔵	Functions	76.21%	3836 / 5033
🔵	Branches	86.71%	8221 / 9480

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
packages/scaffold-ui/src/modal/w3m-modal/styles.ts	100%	100%	100%	100%
packages/ui/src/utils/ThemeConstantsUtil.ts	100%	100%	100%	100%
packages/ui/src/utils/ThemeHelperUtil.ts	53.82%	81.66%	71.42%	53.82%	142-143, 178-179, 193-194, 199-219, 223-251, 260-332, 378-379, 382-383, 386-415, 419-447, 262-461

Generated in workflow #14645 for commit 97f9473 by the Vitest Coverage Report Action