refactor: use object theme to optimize sub system management

[jelychow]

Using object to encapsulate the theme subsystem

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Instructions

No more material themes in apps, this avoids importing different styles so that the theme is always presented as we expect it to be.
colors：NiaTheme.colorScheme.background
custom background：NiaTheme.niaBackground.background
gradient: NiaTheme.gradientColors
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Ensure tests pass and code is formatted correctly

• Run local tests on the DemoDebug variant by running ./gradlew testDemoDebug
• Fix code formatting: ./gradlew --init-script gradle/init.gradle.kts spotlessApply

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Using object to manage Theme's subsystems makes it easier to use them instead of using them directly inside the repo, which violates encapsulation.

NOW DELETE THIS LINE AND EVERYTHING ABOVE IT

What I have done and why

use NiaTheme instead of MaterialTheme
use properties instead of directly using sub systems

• val backgroundColor = NiaTheme.colorScheme.background.toArgb()
• val color = NiaTheme.niaBackground.color
• val tonalElevation = NiaTheme.niaBackground.tonalElevation