fix: fixed X logo in dark mode and removed x.svg file

[DhairyaMajmudar]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Which issue(s) this PR fixes:
Fixes #671

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

1. Fixes the X logo in dark mode by converting the logo into its white gradient while in dark mode.
2. Removed x.svg file as its not used anymore in other parts of codebase

• What is the current behavior? (You can also link to an open issue here)

• What is the new behavior (if this is a feature change)?
  

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  N/A

• Other information:
  N/A

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[DhairyaMajmudar]

@Shelley-BaoYue kindly review pls.

[wbc6080]

Over all looks good, but plz squash some commits https://kubeedge.io/docs/developer/Supplemental_developer_guide#ensuring-to-squash-similiar-commits

[DhairyaMajmudar]

Thank you @wbc6080 for review I've squashed the commits.

[wbc6080]

Thanks for your contributions!
/lgtm