All backgrounds are white

Change-Id: I90c6515c778d425b4358f24e6b55965c871a9f8d
diff --git a/inst/rmarkdown/templates/posterdown_ids/resources/template.html b/inst/rmarkdown/templates/posterdown_ids/resources/template.html
index e7d8938..8f3be50 100644
--- a/inst/rmarkdown/templates/posterdown_ids/resources/template.html
+++ b/inst/rmarkdown/templates/posterdown_ids/resources/template.html
@@ -62,6 +62,7 @@
 text-align: justify;
 font-family: $if(font_family)$$font_family$$else$Fira Sans$endif$;
 hyphens: auto;
+background: white;
 }
 .poster_wrap {
 width: $if(poster_width)$$poster_width$$else$827mm$endif$;