Add Black theme option for battery saving on OLED displays

[timurgilfanov]

Content

Added Theme.Black option alongside the existing Light/Dark/System themes.

Core changes:

• Theme enum gets a new Black value; isDark() returns true for it
• ElementTheme parameter changed from darkTheme: Boolean to theme: Theme; Black theme uses
  compoundDark colors with bgCanvasDefault = Color.Black and derives its Material color scheme from
  that
• ElementThemeApp passes theme directly and maps Theme.Black to MODE_NIGHT_YES

UI:

• New ThemeOption.Black in Advanced Settings
• New string resource common_black = "Black" (in temporary.xml pending Localazy integration)

Housekeeping:

• All darkTheme: Boolean call sites migrated to theme: Theme equivalent
• libraries.designsystem.preview.ElementPreviewBlack added; used in AdvancedSettingsView preview only

Motivation and context

Closes #2624.

Screenshots / GIFs

Tests

• Open Settings
• Open Advanced Settings
• Click on Appearance
• Change it to "Black". Background become pure black.

Tested devices

• Physical
• Emulator
• OS version(s): Android 15

Checklist

• Changes have been tested on an Android device or Android emulator with API 24
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

[github-actions]

Thank you for your contribution! Here are a few things to check in the PR to ensure it's reviewed as quickly as possible:

• Your branch should be based on origin/develop, at least when it was created.
• The title of the PR will be used for release notes, so it needs to describe the change visible to the user.
• The test pass locally running ./gradlew test.
• The code quality check suite pass locally running ./gradlew runQualityChecks.
• If you modified anything related to the UI, including previews, you'll have to run the Record screenshots GH action in your forked repo: that will generate compatible new screenshots. However, given Github Actions limitations, it will prevent the CI from running temporarily, until you upload a new commit after that one. To do so, just pull the latest changes and push an empty commit.

[bxdxnn]

LGTM

[frebib]

I've been using this for a few days and the black with the existing colours looks kinda... bad. The contrast looks off. It's hard to put into words. I think this needs more tweaking before it's shipped

[timurgilfanov]

@frebib thanks for the first hands-on feedback. Could you attach screenshots were you feel most discomforting? And what is the model of your device?

[frebib]

In particular the green banner is too harsh, as is the grey used by the message bubbles is too light. Perhaps they would look better if they are darker? I'm not sure- I'm no UX expert. I just know that dark grey looks better than black with the rest of the colour scheme. If it matters, I'm using a Pixel 8 Pro. I don't imagine screenshots would help because I imagine that it renders the same on any phone.

[timurgilfanov]

I don't imagine screenshots would help because I imagine that it renders the same on any phone.

Screenshots make it easier to make a common understanding what screens and UI elements we talking about. I'm not an active Element user and for me it requires a suggestion to understand what green banner we talking about.

[bmarty]

I am testing the app with a pure Black background, and I think @frebib is talking about the green banner at the top of the screen:

But it's not blocking to me.

I guess we can merge the PR only if the new Black entry is behind a new feature flag (like "Allow black theme"), so that we can let more people test it and give feedback.

[timurgilfanov]

@bmarty I hidden Black theme behind the feature flag.

[bmarty]

Thanks!

[timurgilfanov]

@bmarty looks like at least some of the test failures were unrelated to PR, and will be passed after develop had been merged into pr branch. Can you approve workflows to run?