feat: remove the bor base theme from mobius

[ella-etch]

Removes the BoR base theme from Mobius.
Also removes the CSS variable theme switcher from storybook as there are no additional themes to switch between.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ CSS	stylelint	2		0	1.14s
✅ EDITORCONFIG	editorconfig-checker	6		0	0.03s
✅ JAVASCRIPT	eslint	2		0	3.61s
✅ JSON	jsonlint	2		0	0.23s
✅ JSON	npm-package-json-lint	yes		no	0.39s
✅ JSON	prettier	2		0	1.26s
✅ JSON	v8r	2		0	10.93s
✅ REPOSITORY	checkov	yes		no	16.1s
✅ REPOSITORY	gitleaks	yes		no	7.42s
✅ REPOSITORY	git_diff	yes		no	0.01s
✅ REPOSITORY	secretlint	yes		no	1.39s
✅ REPOSITORY	trivy-sbom	yes		no	2.68s
✅ REPOSITORY	trufflehog	yes		no	4.07s

See detailed report in MegaLinter reports
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