improved README banner

gallery/readme_banner.typ

@@ -1,18 +1,18 @@
 #import "/src/lib.typ": *
 
-#let dark = sys.inputs.at("dark", default: "false") == "true"
+#let theme = sys.inputs.at("theme", default: "default")
 
 #set page(
   width: auto,
   height: auto,
   margin: 1cm,
-  fill: none
+  fill: if theme == "default" {white} else {none}
 )
 
 #set text(
   size: 12pt,
   font: "Source Sans 3",
-  fill: if dark {white} else {black}
+  fill: if theme == "dark" {white} else {black}
 )
 
 #let hex-abc = "0123456789abcdef"