Add change theme function: click the sun icon

[wingsum93]

Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.

Pre-launch Checklist

• I read the rules PR (title, labels, commit messages...).
• I definitely don't PR files with pointless edits.
• I added screenshots if change UI/UX (has modified code in lib/src/ui/*).
• I writen all test cases coverage and all passed.
• All existing and new tests are passing.

Testcases covered (check if passed)

• Manual test change theme by clicking icon

Screenshots (if change UI/UX)

[wingsum93]

This PR fixed the issue in #5

[lambiengcode]

lgtm!