Reduce duplication for the command prompt commands

The `promptCommands.mjs` often reimplement existing functionality to execute in the command. Consider reusing already implemented code

Examples:
- the `ToggleFullscreenPromptCommand` overlaps with the fullscreen button `setupFullscreen` in `Navbar`
- the `ThemePromptCommand` overlaps with `darkmode.js`. Are both necessary or can we reuse code?

Start by reading promptCommands.mjs, then compare ToggleFullscreenPromptCommand with Navbar's setupFullscreen and ThemePromptCommand with darkmode.js. Identify which existing functionality can be shared without changing command behavior; done means the duplicated implementations are reduced and the relevant prompt commands still work.

Written by the indexing model from the issue text.