fix(Makefile): use $(MAKE) for recursive build in build_nc_theming_app

Plain 'make build_css' inside the recipe is the wrong form for a
recursive build: it doesn't inherit the jobserver from the parent make
invocation, so when the Makefile is driven with 'make -j' (or from a
parent build that already holds jobserver tokens) the inner make
either prints 'warning: jobserver unavailable' or silently loses
parallel scheduling.

Replace it with $(MAKE), which GNU Make documents as the canonical
form for recursive invocations — it propagates MAKEFLAGS, jobserver
tokens, and triggers the recursive-make detection that prints
'make[1]: Entering directory ...' on entry.

Caught by GitHub Copilot in PR 109 review:
#109 (comment)

Author: printminion-co

Makefile

@@ -169,7 +169,7 @@ $(EXTERNAL_FULL_TARGETS): build_%_app:
 build_nc_theming_app: ## Build the custom css
 	@echo "[i] Building nc_theming app..."
 	cd apps-custom/nc_theming && \
-	make build_css
+	$(MAKE) build_css
 	@echo "[✓] nc_theming app built successfully"
 
 build_nc-ionos-theme_app: ## Install and build ionos theme