Refactor: CSS handling (#2344)

* refactor(css): only load base styles, add css task, cleanup exports
* style(base): add edge label bg
* refactor(css): use css-utils
* feat(core): add Panel component
* refactor(background): cleanup
* refactor(css-handling): cleanup

tooling/postcss.config.js

@@ -0,0 +1,7 @@
+module.exports = {
+  plugins: [
+    require('postcss-nested'),
+    require('autoprefixer'),
+    require('postcss-import'),
+  ],
+};