ServiceWorkerInternals: Update to use CrElements and support dark mode.

This change refactors the `chrome://serviceworker-internals` page to use
`<cr-button>` and `<cr-checkbox>` from `cr_elements`. It also updates
the styling to use CSS variables for colors, providing better
consistency and adding dark mode support.

Bug: 442444722
Change-Id: I067859c0e28a2e529366cfcd8baef025d0560de8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6907627
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Patrick Kettner <patrickkettner@google.com>
Reviewed-by: John Lee <johntlee@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Patrick Kettner <patrickkettner@google.com>
Cr-Commit-Position: refs/heads/main@{#1517971}