Dynamically register which CSS files are needed.

src/context/dependency_manager.rs

@@ -65,4 +65,20 @@ impl DependencyManager {
         std::mem::swap(&mut self.dependencies, &mut dependencies);
         dependencies
     }
+
+    pub(crate) fn include_css<N>(&mut self, name: N) -> Result<(), CustomError>
+    where
+        std::string::String: From<N>,
+    {
+        self.dependencies
+            .push(Dependency::CssFile { name: name.into() });
+        Ok(())
+    }
+
+    pub(crate) fn list_css(&self) -> Result<impl Iterator<Item = &String>, CustomError> {
+        Ok(self.dependencies.iter().filter_map(|dep| match dep {
+            Dependency::CssFile { name } => Some(name),
+            _ => None,
+        }))
+    }
 }