Glad that you like it :-)

Possible features/changes are listed in the page at https://github.com/webextensions/live-css-editor/blob/master/TODO.md

External window:
Implementing that would be a bigger change. There is an existing workaround which can solve the use-case of editing in responsive design view to an ok-enough level. You can use the feature "Watch CSS files to apply changes automatically" and follow the instructions there to set it up. After that, open your file in any code editor and edit it. You changes would reflect immediately.

Providing light/dark theme:
That may need an alternative copy of the icons and the UI for it. Since that is extra effort, I'm currently lazy about it ;-)

Changing the background color to neutral dark-gray with appropriate shadow:
That can be done.

I've added your suggestions in the TODO file mentioned above, but may not be taking them up soon due to other listed priority tasks or time constraints. PRs are welcome :-)