Add build script to make an (svg) demo of all available themes

Makefile

@@ -21,3 +21,7 @@ test: fmt
 .PHONY: race
 race: fmt
 	prefix "$@" ./ci/test.sh --race ./...
+
+.PHONY: themesdemo
+themesdemo:
+	$(MAKE) -C ./testdata/examples/

testdata/examples/Makefile

@@ -0,0 +1,14 @@
+D2 := ../../d2
+
+SVGS = $(shell $(D2) themes | gawk -F':' '/^-/{ printf "out/themex-%03d.svg ",$$2 }' || :)
+
+.PHONY:
+all: $(D2) $(SVGS)
+
+$(D2):
+	cd $(dir $@) && go build
+	$(MAKE) $(SVGS)
+# D2 as dependency to expire results if recompiled
+out/themex-%.svg: themex.d2 $(D2)
+	$(info Building $@ from $< ...)
+	$(D2) -t $$(( 10#$* )) $< $@

testdata/examples/themex.d2

@@ -0,0 +1,50 @@
+
+vars: {
+  d2-config: {
+    layout-engine: elk
+  }
+}
+network: {
+  cell tower: {
+    satellites: {
+      shape: stored_data
+      style.multiple: true
+    }
+
+    transmitter
+
+    satellites -> transmitter: send
+    satellites -> transmitter: send
+    satellites -> transmitter: send
+  }
+
+  online portal: {
+    ui: {shape: hexagon}
+  }
+
+  data processor: {
+    storage: {
+      shape: cylinder
+      style.multiple: true
+    }
+  }
+
+  cell tower.transmitter -> data processor.storage: phone logs
+}
+
+user: {
+  shape: person
+  width: 130
+}
+
+user -> network.cell tower: make call
+user -> network.online portal.ui: access {
+  style.stroke-dash: 3
+}
+
+api server -> network.online portal.ui: display
+api server -> logs: persist
+logs: {shape: page; style.multiple: true}
+
+network.data processor -> api server
+