Dynamic Theming for Angular Applications

This challenge focuses on implementing a robust and flexible dynamic theming system within an Angular application. The ability to switch themes on the fly allows for greater user customization, accessibility options, and brand consistency across different environments. You will build a system that enables users to select and apply different visual themes to the application without requiring a page reload.

Problem Description

Your task is to create a dynamic theming solution for an Angular application. This system should allow for multiple predefined themes to be loaded and applied. The selected theme should persist across user sessions (e.g., using local storage) and be switchable at runtime.

Key Requirements:

Theme Definition: Define at least two distinct themes (e.g., "light" and "dark"). Each theme should consist of a set of CSS variables or styles that alter the visual appearance of common UI elements (e.g., background color, text color, button styles, primary/secondary colors).
Theme Management Service: Create an Angular service responsible for managing the current theme, loading themes, and providing a mechanism to switch between them.
Theme Application: Implement a strategy to apply the selected theme to the entire application. This could involve dynamically adding/removing CSS classes to the <body> tag or injecting a <style> tag with the theme's CSS.
Persistence: The selected theme should be stored (e.g., in localStorage) so that it persists when the user navigates away from the page or closes and reopens the browser.
UI Control: Provide a simple UI element (e.g., a dropdown or buttons) that allows the user to select and switch between available themes.
Integration: Ensure that existing or new components within the application correctly reflect the applied theme.

Expected Behavior:

When the application loads, it should either apply the last selected theme or a default theme if no theme has been previously selected.
Users can interact with a UI element to change the current theme.
Upon theme selection, the application's visual styles should update immediately without a page refresh.
The selected theme should be preserved across browser sessions.

Edge Cases to Consider:

What happens if localStorage is not available or disabled?
How are initial styles loaded before the theme is applied?
How can you ensure new components added to the application automatically inherit the current theme?

Examples

Example 1: Initial Load (No prior theme)

Input: User opens the application for the first time.
Output: The application loads with the default theme (e.g., "light"). The localStorage is empty.
Explanation: Since no theme is stored, the application defaults to a predefined theme.

Example 2: Theme Switching

Input: User is viewing the application with the "light" theme and clicks a button to switch to the "dark" theme.
Output: The application's background changes to dark, text becomes light, and any other theme-dependent styles are updated. The "dark" theme name is stored in localStorage.
Explanation: The theme management service detects the change, applies the "dark" theme's styles, and saves the preference for future use.

Example 3: Persistence on Reload

Input: User has previously selected the "dark" theme, closes the browser, and then reopens the application.
Output: The application loads with the "dark" theme already applied.
Explanation: The theme service reads the "dark" theme preference from localStorage upon initialization and applies it.

Constraints

The solution must be implemented using Angular and TypeScript.
At least two distinct themes must be supported.
Theme switching should be performant and avoid unnecessary re-renders.
The chosen method for applying themes should be scalable.

Notes

Consider using CSS custom properties (variables) for defining theme styles. This makes it easy to override styles dynamically.
Think about how to structure your theme definitions (e.g., separate CSS files, JSON objects).
The Renderer2 service in Angular can be helpful for manipulating DOM elements and attributes.
For theme persistence, window.localStorage is a common choice, but consider fallback mechanisms.
How will you handle theming of third-party UI components if you were to use them? (This is an advanced consideration, not strictly required for this challenge but good to think about).

Dynamic Theming for Angular Applications

Problem Description

Key Requirements:

Theme Definition: Define at least two distinct themes (e.g., "light" and "dark"). Each theme should consist of a set of CSS variables or styles that alter the visual appearance of common UI elements (e.g., background color, text color, button styles, primary/secondary colors).

Theme Management Service: Create an Angular service responsible for managing the current theme, loading themes, and providing a mechanism to switch between them.

Theme Application: Implement a strategy to apply the selected theme to the entire application. This could involve dynamically adding/removing CSS classes to the <body> tag or injecting a <style> tag with the theme's CSS.

Persistence: The selected theme should be stored (e.g., in localStorage) so that it persists when the user navigates away from the page or closes and reopens the browser.

UI Control: Provide a simple UI element (e.g., a dropdown or buttons) that allows the user to select and switch between available themes.

Integration: Ensure that existing or new components within the application correctly reflect the applied theme.

Expected Behavior:

When the application loads, it should either apply the last selected theme or a default theme if no theme has been previously selected.

Users can interact with a UI element to change the current theme.

Upon theme selection, the application's visual styles should update immediately without a page refresh.

The selected theme should be preserved across browser sessions.

Edge Cases to Consider:

What happens if localStorage is not available or disabled?

How are initial styles loaded before the theme is applied?

How can you ensure new components added to the application automatically inherit the current theme?

Examples

Example 1: Initial Load (No prior theme)

Input: User opens the application for the first time.

Output: The application loads with the default theme (e.g., "light"). The localStorage is empty.

Explanation: Since no theme is stored, the application defaults to a predefined theme.

Example 2: Theme Switching

Input: User is viewing the application with the "light" theme and clicks a button to switch to the "dark" theme.

Output: The application's background changes to dark, text becomes light, and any other theme-dependent styles are updated. The "dark" theme name is stored in localStorage.

Explanation: The theme management service detects the change, applies the "dark" theme's styles, and saves the preference for future use.

Example 3: Persistence on Reload

Input: User has previously selected the "dark" theme, closes the browser, and then reopens the application.

Output: The application loads with the "dark" theme already applied.

Explanation: The theme service reads the "dark" theme preference from localStorage upon initialization and applies it.

Notes

Consider using CSS custom properties (variables) for defining theme styles. This makes it easy to override styles dynamically.

Think about how to structure your theme definitions (e.g., separate CSS files, JSON objects).

The Renderer2 service in Angular can be helpful for manipulating DOM elements and attributes.

For theme persistence, window.localStorage is a common choice, but consider fallback mechanisms.

How will you handle theming of third-party UI components if you were to use them? (This is an advanced consideration, not strictly required for this challenge but good to think about).