Implementing Style Isolation in Angular Components

Angular's component architecture inherently provides a degree of style isolation, but sometimes you need more robust control to prevent styles from bleeding between components, especially when using global CSS or shared style sheets. This challenge asks you to implement a mechanism to ensure that styles defined within a component are truly isolated and do not affect other components, even when using a shared stylesheet. This is crucial for maintainability and preventing unexpected styling conflicts in larger applications.

Problem Description

You need to create an Angular component that utilizes a shared stylesheet but guarantees style isolation. The component should:

1. Utilize a Shared Stylesheet: The component will import a stylesheet named shared.css (provided in the setup). This stylesheet contains some basic styles that you want to apply to multiple components.
2. Implement Style Isolation: Despite importing shared.css, the styles defined within this component should not leak out and affect other components. Similarly, styles defined in other components should not affect this component beyond what's explicitly defined in shared.css.
3. Apply Component-Specific Styles: The component should also have its own, unique styles that are only applied to this component's template.
4. Dynamic Styling: The component should accept an input property called theme (string type) and dynamically apply a class based on the value of this property. The possible values for theme are "light" and "dark". The component should apply the class light-theme when theme is "light" and dark-theme when theme is "dark".

Expected Behavior:

• When the component is rendered, it should display content styled according to both shared.css and its own component-specific styles.
• Changes to the theme input property should dynamically update the component's appearance by adding or removing the appropriate theme class.
• Styles defined in other components should not affect the styling of this component, and vice versa (beyond the styles defined in shared.css).

Edge Cases to Consider:

• What happens if the theme input is not "light" or "dark"? The component should gracefully handle this (e.g., by not applying any theme class).
• How does Angular's scoping mechanism interact with the shared stylesheet?

Examples

Example 1:

Input:
Component Input: theme = "light"
shared.css:
.my-element { color: blue; }
.light-theme { background-color: white; }
Component-Specific Styles:
.my-element { font-weight: bold; }

Output:
A "my-element" with blue text, bold font, and a white background.  Other components will not have a white background.
Explanation: The shared styles apply the blue color. The component-specific styles make the text bold. The "light-theme" class adds the white background, isolated to this component.

Example 2:

Input:
Component Input: theme = "dark"
shared.css:
.my-element { color: blue; }
.dark-theme { background-color: black; }
Component-Specific Styles:
.my-element { font-size: 16px; }

Output:
A "my-element" with blue text, a font size of 16px, and a black background. Other components will not have a black background.
Explanation: The shared styles apply the blue color. The component-specific styles set the font size. The "dark-theme" class adds the black background, isolated to this component.

Example 3:

Input:
Component Input: theme = "invalid"
shared.css:
.my-element { color: blue; }
.light-theme { background-color: white; }
.dark-theme { background-color: black; }
Component-Specific Styles:
.my-element { padding: 10px; }

Output:
A "my-element" with blue text, 10px padding, and no background color.
Explanation: The shared styles apply the blue color. The component-specific styles add padding.  No theme class is applied because the input is invalid.

Constraints

• Angular Version: Assume Angular version 14 or higher.
• CSS Approach: You must use component-level styles (encapsulation) provided by Angular. Do not use shadow DOM directly.
• Stylesheet Location: The shared.css file will be located in the same directory as the component's TypeScript file.
• Performance: The solution should be performant and avoid unnecessary DOM manipulations.
• Input Type: The theme input must be a string.

Notes

• Angular's component encapsulation is key to solving this problem. Understand how Angular handles CSS scoping.
• Consider how to dynamically add and remove CSS classes based on the theme input.
• The shared.css file will be provided as part of the setup. You do not need to create it. It will contain basic styles.
• Focus on ensuring that styles are truly isolated and do not bleed between components.