refactor[devtools]: update css for settings and support css variables in shadow dom scnenario (#33487)

## Summary

Minor changes around css and styling of Settings dialog.

1. `:root` selector was updated to `:is(:root, :host)` to make css
variables available on Shadow Root
2. CSS tweaks around Settings dialog: removed references to deleted
styles, removed unused styles, ironed out styling for cases when input
styles are enhanced by user agent stylesheet

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## How did you test this change?

| Before | After |
|--------|--------|
| ![Screenshot 2025-06-09 at 15 35
55](https://github.com/user-attachments/assets/1ac5d002-744b-4b10-9501-d4f2a7c827d2)
| ![Screenshot 2025-06-09 at 15 26
12](https://github.com/user-attachments/assets/8cc07cda-99a5-4930-973b-b139b193e349)
|
| ![Screenshot 2025-06-09 at 15 36
02](https://github.com/user-attachments/assets/1af4257c-928d-4ec6-a614-801cc1936f4b)
| ![Screenshot 2025-06-09 at 15 26
25](https://github.com/user-attachments/assets/7a3a0f7c-5f3d-4567-a782-dd37368a15ae)
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| ![Screenshot 2025-06-09 at 15 36
05](https://github.com/user-attachments/assets/a1e00381-2901-4e22-b1c6-4a3f66ba78c9)
| ![Screenshot 2025-06-09 at 15 26
30](https://github.com/user-attachments/assets/bdefce68-cbb5-4b88-b44c-a74f28533f7d)
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| ![Screenshot 2025-06-09 at 15 36
12](https://github.com/user-attachments/assets/4eda6234-0ef0-40ca-ad9d-5990a2b1e8b4)
| ![Screenshot 2025-06-09 at 15 26
37](https://github.com/user-attachments/assets/5cac305e-fd29-460c-b0b8-30e477b8c26e)
|