Define the theme to be used and center images

Makefile

@@ -4,7 +4,7 @@
 help:
 	@echo "Please use \`make <target>' where <target> is one of"
 	@echo "  pdf        to make standalone PDF files"
-	
+
 clean:
 	rm -rf $(BUILDDIR)/*
 
@@ -14,4 +14,4 @@ html:
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
 pdf:
-	asciidoctor-pdf -a pdf-themesdir=resources/themes -a pdf-theme=gwift-theme source/main.adoc -t -r asciidoctor-diagram
+	asciidoctor-pdf -a pdf-themesdir=resources/themes -a pdf-theme=book.yml source/main.adoc -t

resources/themes/book.yml

@@ -11,4 +11,7 @@ admonition:
     caution:
       name: fa-fire
       stroke_color: ff0000
-      size: 24
+      size: 24
+image:
+  caption:
+    align: inherit