Try to fix the paths.

app/assets/stylesheets/pdf.css.scss

@@ -8,7 +8,10 @@ a {
 
 // We need the import here, otherwise Rails' asset adds a BOM at the beginning of it,
 // and the font doesn't get recognize!
-@import "@cubing/icons"; /* stylelint-disable-line no-invalid-position-at-import-rule */
+// 
+// This refers to: `[repo root]/node_modules/@cubing/icons/dist/lib/@cubing/icons/cubing-icons.css`,
+// per a configuration in `assets.rb`.
+@import "cubing-icons"; /* stylelint-disable-line no-invalid-position-at-import-rule */
 
 header {
   width: 100%;

config/initializers/assets.rb

@@ -10,7 +10,13 @@
 # helper!
 # wicked_pdf's helpers can't handle "packs_with_chunks" and therefore cannot
 # properly inline our packs :(
-Rails.application.config.assets.paths << Rails.root.join("node_modules", "@cubing", "icons", "www", "css")
+# 
+# Note: reaching into the package structure violates the semantics of package
+# exports, which could theoretically turn any release of `@cubing/icons` into a
+# breaking one. However, this path should be fairly stable, and the usage here
+# should be safe as long as `@cubing/icons` is not bumped automatically without
+# exercising this code path in a test.
+Rails.application.config.assets.paths << Rails.root.join("node_modules", "@cubing", "icons", "dist", "lib", "@cubing", "icons")
 
 # Precompile additional assets.
 # application.js, application.css, and all non-JS/CSS in app/assets folder are already added.