shift asset processing to webpack (#424)

also migrated SASS to latest syntax (via dart-sass) and vastly simplified light/dark theme logic

.gitignore

@@ -4,6 +4,10 @@ builds/
 _vendor/
 hugo_stats.json
 
+# webpack artifacts
+data/manifest.json
+static/assets/
+
 # node/npm/yarn
 node_modules/
 package-lock.json