Make Shoelace follow the computed color-scheme of the host page

Generated with the help of an LLM.

Problem/Motivation

Follow-up of #3633745: Avoid light/dark mismatch between parts of the UI. Shoelace turns dark on two classes of <html>: dark-mode from Default Admin and gin--dark-mode from Gin.
Any other theme with a dark mode keeps a light builder, as a UI Suite theme hosting the builder when the site sets no admin theme.
UI Suite Bootstrap sets its dark mode on <body>, so the computed color-scheme of <html> stays normal.

Proposed resolution

Read the computed color-scheme of <body> instead of the class list. A pick in the Theme menu still wins.

Remaining tasks

• Decide what to observe for a change of mode: the attributes of <html> and <body>, or the prefers-color-scheme media query for a theme in System mode.
• Check Default Admin, Gin, Claro, UI Suite Bootstrap and UI Suite DSFR as host theme, light and dark.

User interface changes

None.

API changes

None.

Data model changes

None.