add theme picker

lib/html_utils.typ

@@ -1,4 +1,16 @@
+#let css-list = state("css-list", ())
+
+/// Add string `css` to `css-list` if not already present
+#let add-css(css) = context {
+  css-list.update(x => if css in x { x } else { x + (css,) })
+}
+
+/// Concatenate all css found in css-list at the end of the document
+#let get-css() = context {
+  css-list.final().join("\n\n")
+}
+
 #let html_show(body) = {
-  show raw.where(lang: "raw-css"): it => html.style(it.text)
+  show raw.where(lang: "raw-css"): it => add-css(it.text)
   body
 }