fix: replace undeclared body variable with document.body in theme preference loading

[AdityaM-IITH]

What:

• Replaced two references to undeclared variable body with document.body
  in the theme preference loading logic in the Activity constructor.

Why:

• body was never declared in this scope, causing a ReferenceError silently
  swallowed by the surrounding try/catch block.
• This meant stored theme preference was never actually applied on load.
• document.body is already used elsewhere in this file, making this the
  correct and consistent reference.

How:

• Minimal two-line change, no logic modification.

[github-actions]

❌ Some Jest tests failed. Please check the logs and fix the issues before merging.

Failed Tests:

palette.test.js
logo.test.js
logoconstants.test.js

[AdityaM-IITH]

Rebased on latest master (83c5c8d). All 117 test suites passing locally.

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.