Left border tracks tool lifecycle; hover brightens for interactivity.
DEFAULT_CSS = '\n ToolCallMessage {\n height: auto;\n padding: 0 1;\n margin: 0 0 1 0;\n background: transparent;\n border-left: wide $tool;\n }\n\n ToolCallMessage .tool-header {\n height: auto;\n color: $tool;\n text-style: bold;\n }\n\n ToolCallMessage .tool-task-desc {\n color: $text-muted;\n margin-left: 3;\n text-style: italic;\n }\n\n ToolCallMessage .tool-args {\n color: $text-muted;\n margin-left: 3;\n }\n\n ToolCallMessage .tool-status {\n margin-left: 3;\n }\n\n ToolCallMessage .tool-status.pending {\n color: $warning;\n }\n\n ToolCallMessage .tool-status.success {\n color: $success;\n }\n\n ToolCallMessage .tool-status.error {\n color: $error;\n }\n\n ToolCallMessage .tool-status.rejected {\n color: $warning;\n }\n\n ToolCallMessage .tool-output {\n margin-left: 0;\n margin-top: 0;\n padding: 0;\n height: auto;\n }\n\n ToolCallMessage .tool-output-preview {\n margin-left: 0;\n margin-top: 0;\n }\n\n ToolCallMessage .tool-output-hint {\n margin-left: 0;\n color: $text-muted;\n }\n\n ToolCallMessage:hover {\n border-left: wide $tool-hover;\n }\n '