Fix #3782 : Made the project use Themes instead of Styles [Dark Mode Implementation].

[ayush0402]

Explanation

Fixes #3782
Most part of the existing styles.xml already followed suggested practices for themes.xml according to https://medium.com/androiddevelopers/android-styling-themes-vs-styles-ebe05f917578 with semantic naming for color resources.
Separated certain styles suited more for themes.xml from styles.xml.

Possible solution for further PRs :
Adding themes.xml and colors.xml under values-night and changing theme's parent to DayNight variant with changes under values-night for proper implementation of Dark Mode.

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

[ayush0402]

@rt4914 PTAL If this approach is acceptable.

[rt4914]

@ayush0402 Nit changes suggested. @BenHenning Can you confirm if this approach is correct.

[BenHenning]

Took a first pass, but I'd like to take a closer look tomorrow. I think this generally looks right, though.

[BenHenning]

Thanks @ayush0402. Took a closer look. PR generally LGTM, but had a couple of comments--PTAL.

[rt4914]

LGTM

[ayush0402]

@BenHenning PTAL.

[BenHenning]

Thanks @ayush0402. This LGTM!