The _TextualCleverAgentsTuiApp class was missing the THEME class
variable, causing Textual to use its default textual-dark theme
instead of the Dracula theme required by the spec (docs/specification.md
§TUI Theme). Added THEME: ClassVar[str] = "dracula" to the class and
a corresponding BDD scenario tagged @tdd_issue @tdd_issue_4742 to
verify the fix.
ISSUES CLOSED: #4742