add test for rustdoc's --themes flag

src/test/run-make-fulldeps/rustdoc-themes/Makefile

@@ -0,0 +1,10 @@
+-include ../tools.mk
+
+# Test that rustdoc will properly load in a theme file and display it in the theme selector.
+
+OUTPUT_DIR := "$(TMPDIR)/rustdoc-themes"
+
+all:
+	cp $(S)/src/librustdoc/html/static/themes/light.css $(TMPDIR)/test.css
+	$(RUSTDOC) -o $(OUTPUT_DIR) foo.rs --themes $(TMPDIR)/test.css
+	$(HTMLDOCCK) $(OUTPUT_DIR) foo.rs

src/test/run-make-fulldeps/rustdoc-themes/foo.rs

@@ -0,0 +1,4 @@
+// @has test.css
+// @has foo/struct.Foo.html
+// @has - '//link[@rel="stylesheet"]/@href' '../test.css'
+pub struct Foo;