theme_importer: Output logs to stderr (#14890)

Will allow writing directly to a file without logging via `cargo run -p
theme_importer -- /path/to/file`

Release Notes:

- N/A

crates/theme_importer/src/main.rs

@@ -106,7 +106,7 @@ fn main() -> Result<()> {
     TermLogger::init(
         LevelFilter::Trace,
         log_config,
-        TerminalMode::Mixed,
+        TerminalMode::Stderr,
         ColorChoice::Auto,
     )
     .expect("could not initialize logger");