chore: remove fallback css variables for MobileNavItem and amend the stories

[ss-dimasm]

Pull request checklist

Detail as per issue below (required):

fixes: #206

addition amendment to not store the fallback CSS variable values in MobileNavItem, and amend the story files to able show the correct example on how to use in React usage

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 8e5182c1	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (8e5182c)	Report Missing	Report Missing	Report Missing
Head commit (bb068b1)	4512	4054	89.85%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#255)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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