Accessibility Bug Fix: Tab focus doesn't reach Dark mode toggle button

[bhavykhatri]

Description

This is a fix for accessibility bug

• if you press tab then the focus doesn't reach dark mode toggle button
• After focus should reach the dark mode toggle button, pressing enter button should change the theme of website
  

e.g. Fixes #(issue)
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Type of change

• 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)
• Documentation update

How has this been tested

• I have run bash ./tools/deploy.sh --dry-run (at the root of the project) locally and passed
• I have tested this feature in the browser

Test Configuration

• Browser type & version:
• Operating system: Windows
• Ruby version: 2.7.4p191
• Bundler version: 2.2.30
• Jekyll version: 4.2.1

Checklist

• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[bhavykhatri]

@cotes2020