feat(docs): add dark theme mode

[gautamgambhir97]

Proposed Changes

add dark theme mode

Linked Issues

[if applicable, add links to issues resolved by this PR]

Types of changes

What type of change does this pull request make (put an x in the boxes that apply)?

• Content update.
• Bug fix (non-breaking change that fixes an issue).
• New feature added (non-breaking change that adds functionality).
• Breaking change (fix or feature that would cause existing functionality to stop working as expected).
• Something else (e.g., tests, scripts, example, deployment, infrastructure).

Checklist

Put an x in the boxes that apply:

• I have read the CONTRIBUTING guide
• Checks and tests pass locally

Further comments

[if this is a relatively large or complex change, kick off a discussion by explaining why you chose the solution you did, what alternatives you considered, etc...]